Urine Sodium and Fractional excretion of Sodium (FENa)

Urine Sodium and FENa:

Urine sodium is the concentration of sodium in the urine. Remember that the normal value for blood sodium is 135-145 mmol/litre. Note that the range of sodium in urine is far greater than the range in the blood.

The reference range varies according to age and sex. If also depends upon the hydration state of the person. 

  40-220 mmol/L (For male)
27-287 mmol/L (For female)

For children aged 6-10 Year:
41-115 (For male)
20-69 (For female)

Dipstick method for Urinalysis (MULTISTIX METHOD)

DIPSTICK METHOD OF URINALYSIS

Multistix reagent strip method

Multistix reagents are clear plastic strips. Seven different reagent areas are affixed on the strip. These different cellulose areas are impregnated with specific testing chemicals according to the test which reacts with specific substances present in urine by changing the color. Color change chart is observed and compared to the color chart for the presence of abnormal levels of substances .Special care in their use is required to prevent inaccurate results and confirmation of quantitative test is appropriate if the results from dipstick testing reveal abnormalities.

The various determinations done by multistix are

pH, Specific Gravity, Glucose, Protein, Ketones, Urobilinogen, Blood, Bilirubin

Fig. Multiple strip for Urinalysis and their interpretaiton

The reagent strips are also available for only one or two tests such as glucose, glucose and protein, glucose and ketones.

Urinalysis : Chemical Examination

Chemical Examination of Urine

The routine analysis of urine includes chemical test for protein, glucose, ketone bodies, occult blood, bile salts, bile pigments and urobilinogen. 

Proteins in urine

Urine normally contains only a scant amount of protein which derives both from blood and urinary tract itself. Mainly albumin is filtered from nephrons due to low molecular weight others are reabsorbed by renal tubules. Other protein includes serum or plasma globulin, mucus or mucin, hemoglobin, bence jones protein.

Determination of protein in urine

Principle

All the methods are based on the principle of precipitation of protein by chemical agents or coagulation by heat.

Urinalysis: Collection, Preservation and Physical Examination

Introduction

Urine is an ultra-filtrate of plasma from which substances essential to the body are reabsorbed and through which those substances not needed are excreted. The nature and amount of substances present in urine reflect ongoing physiological processes in health and disease states.

Routine urine analysis is mainly performed for two purposes: 

• To find out the metabolic and endocrine disturbances of body
• To detect the intrinsic condition that may adversely affect the urinary tract or the kidney.

Collection of urine

For urine analysis fresh early morning sample is preferred because it is most concentrated and acidic which preserves the formed elements well. For the quantitative test and concentration test for tubercle bacilli 24 hour urine specimen is required. For bacteriological test collected urine sample should be free from contamination and it should be collected in sterile container .collection by catheterization into sterile container is the best.

Tubular proteinuria

Tubular proteinuria

Unlike glomerular proteinuria, where protein excretion can reach 20g/24h and consists mainly of albumin, tubular proteinuria is generally less than 1-2 g/day. Along with albumin there is also larger proportion of proteins of <60 kDa.

Protein	Normal	Glomerular disease	Tubular disease
Total protein (g/day)	<0.15	>2.5	<0.5-1.0
Albumin (gm/day)	<20	>500	<500
α1-microglobulin (mg/day)	<10	Normal/slightly increased	Moderately increased
Β2-microglobulin (mg/24h)	<0.35	Normal/slightly increased	Moderately increased

In renal tubular disease reabsorption of protein together with water, ions, glucose and amino acids is impaired. In tubular disease increase in excretion of low molecular weight protein is greater than that found for larger proteins like albumin e.g. higher clearance of β₂-microglobulin. The albumin to β₂-microglobulin ratio becomes 1:2. 

During this condition there is direct tubular damage mediated by autoimmune or infection, drugs (phenacetin, paracetamol, NSAID), toxins and any pathology leading to glomerular proteinuria. Tubular proteinuria, together with defective reabsorption may form part of the Debre-de Toni-Fanconi syndrome which can be inherited or acquired as a consequence of other inherited metabolic defects.

Increased β₂-microglobulin excretion was first described in cadmium workers, and markers of tubular proteinuria are now used to monitor occupationally exposed subjects. Many drugs like NSAIDS, paracetamol can also cause glomerular and tubular damage.

Methods of assessing tubular damage:

Measurement of urinary proteins as markers of tubular damage falls into two categories: measurement of large molecular weight proteins such as enzymes or brush border antigens released by damaged tubules, and measurement of low MW proteins filtered at glomerulus and normally reabsorbed by PCT.

High molecular weight protein markers of renal tubular damage:

Renal tubular damage causes the release of enzymes not normally filtered by glomerulus and these are lactate dehydrogenase, GGT and ALP. The most widely employed for monitoring tubular damage is N-acetyl β-D-glucosaminidase (NAG) which is a lysosomal enzyme (150 kDa) found in PCT cells. It has been used as marker of acute rejection in renal transplant patients. Its release is increased during nephrotic syndrome, tubulointerstitial nephritis, metal poisoning, DM, hyperthyroidism, etc.

Low molecular weight protein markers of renal tubular disease:

During glomerular disease, tubules are presented with increased load of Low MW proteins which exceed the absorption threshold of tubules. These are retinol binding protein (21 kDa), α₁-microglobulin (31 kDa), β₂-microglobulin (11.8 kDa, this is unstable at urine pH<5.5). Lysozyme (muramidase) was the first Low MW marker of tubular function but its concentration increases during leukemia, inflammation. Increased excretion of β₂-microglobulin was first identified in patients with cadmium poisoning and with Wilson’s disease but increased level are also found in liver disease and malignancies like myeloma and B-cell lymphomas. RBP (85 kDa) complexes with vitamin A and prealbumin in plasma and only small proportion is free to be filtered by glomerulus. RBP is also excreted by kidneys and is stable in urine. α₁-microglobulin is more stable in urine than β₂-microglobulin and is widely used tubular marker.

Proteinuria of prerenal origin:

Proteinuria of prerenal origin has been defined as the occurrence in the urine of abnormal amounts of any plasma protein filtered by glomerulus in the absence of any glomerular or tubular abnormality. The term is applied to overflow proteinuria such as Bence Jones proteinuria, hemoglobinuria, myoglobinuria where plasma concentrations of these proteins are increased.

Myoglobinuria causes during rapid destruction of striated muscle (rhabdomyolysis) which releases myoglobin. As its molecular weight is 17 kDa it is filtered rapidly by glomerulus.

Haemoglobinuria occurs during intravascular hemolysis. In case of myoglobinuria and hemoglobinuria, these are degraded and there is formation of ferrihaemate within the tubules which damages tubular epithelial cells.

Bence Jones proteinuria, the presence in urine of immunoglobulin light chains is frequent finding in multiple myeloma and other B-cell malignancies. Normally kidneys catabolizes the light chain but in excess light chains are filtered, they are toxic to both proximal and distal renal tubules and contribute to development of Fanconi syndrome, renal tubular acidosis and renal failure in patients with multiple myeloma. 

Pathological consequences of glomerular proteinuria:

Pathological consequences of glomerular proteinuria:

Hypoalbuminaemia: 

Hypoalbuminemia found in nephrotic syndrome is due to urinary loss exceeding hepatic synthesis. Also due to increased loss in urine, the absorbed albumin is catabolized in renal tubule cells (PCT) where much of filtered albumin is reabsorbed. Also loss of fixed anionic charge may extend to endothelium of all capillaries leading to generalized increase in capillary permeability to albumin in addition to increased glomerular filtration.

Oedema and salt and water retention: 

Oedema occurs due to hypoalbuminaemia which decreases oncotic pressure and sieving of fluid for intravascular space. The resulting hypovolaemia stimulate the renin-aldosterone system and vasopressin release, leading to sodium and water retention.

Abnormalities of other plasma proteins: 

Patients with nephrotic syndrome have changes in the concentrations of circulating clotting factors which increases the risk of thrombus formation. Urinary loss of natural anticoagulant antithrombin III is probably involved together with increased plasma concentration of fibrinogen and factors V, VII, VIII and IX. Urinary loss of vitamin D and vitamin D-binding globulin may lead to vitamin D deficiency with low plasma ionized calcium, secondary hyperparathyroidism and renal osteodystrophy. Patients with NS also loose Igs and complement components with attendant increased risk of infections like pleurisy, pneumococcal pneumonia and peritonitis.

Hyperlipidaemia: 

Patients with NS has increased cholesterol and Tg which correlate inversely with serum concentration of albumin. VLDL and LDL are increased, partly due to reduced clearance and partly to increased hepatic synthesis, while HDL is reduced may be due to increased urinary loss. VLDL and LDL are apoB containing lipoproteins which are atherogenic that results in glomerular and interstitial renal disease. 

The activities of LCAT and lipoprotein lipases are reduced in NS which could reduce HDL production. Hepatic synthesis of VLDL and LDL is stimulated by reduction in plasma colloidal osmotic pressure, since treatment with albumin or dextran infusions reduces hepatic lipoprotein synthesis. Nephrotic hyperlipidemia is accompanied by an increased risk of cardiovascular complications and is usually treated aggressively with statins.

Massive proteinuria – Nephrotic syndrome >3.5g/day, usually no hematuria, hypoalbuminemia, hyperlipidemia, edema, hypo or normotensive, GFR normal or increased.

Moderate proteinuria – Nephritic syndrome (acute nephritis), acute renal failure.

Asymptomatic proteinuria (or transient) – Proteinuria <1g/day, no other abnormalities, probably orthostatic, proteinuria >2g/day possible early glomerulonephritis

Persistent low level proteinuria/microabluminuria – Associated with early diabetic nephropathy, tubulointerstitial disease, early CVD, hypertension.

Glomerular proteinuria – Nephrotic syndrome:

Glomerular proteinuria – Nephrotic syndrome:

Albuminuria occurs if albumin excretion is >3.0g/day and losses above this are usually due to increased glomerular permeability associated with glomerular damage. Glomerulonephritis is immunologically mediated diseases and excludes other conditions associated with glomerular damage as in diabetes mellitus or amyloidosis.

Minimal change glomerulonephritis:

This is also a type of nephrotic syndrome. During this condition there is little or no abnormality and no immunoglobulin or complement components. Proteinuria involves loss of fixed negative charge on glomerular basement membrane due to fusion of epithelial cell foot processes. This leads to selective proteinuria (mainly albumin). Here T-cell dysfunction may play a role. In this condition there is good long-term prognosis with preserved renal function as all cases respond to treatment (steroid-responsive nephrotic syndrome). It is common in children.

Membranous glomerulonephritis: 

It is the commonest cause of nephrotic syndrome in adults. The cause may be idiopathic or associated with inflammatory or neoplastic disease. There is thickening of glomerular basement membrane. There is IgG deposits seen most commonly. It may also be associated with SLE where immune complexes formed in circulation deposits in GBM. There is massive proteinuria and poorly selective and some have asymptomatic proteinuria or microscopic hematuria. Proposed hypothesis is the role of extrinsic antigen deposition in the glomerulus with subsequent IC formation, glomerular trapping of circulating IC or antibodies binding to intrinsic glomerular antigens. Currently treatment is aimed at lowering blood pressure with ACE inhibitors or angiotensin II receptor antagonists which reduces proteinuria over six months. If proteinuria persists immunosuppressive treatment is generally started e.g. cyclosporine, cyclophosphamide, etc.

Proliferative glomerulonephritis: 

Here glomeruli appear hypercellular, due, for example, to invading macrophages or mesangial cells. Different disease can elicit immunologically mediated glomerular damage, including bacterial endocarditis, Group A streptococcal infection and SLE. There is frequent IgA, IgG and C3 deposits, and complement activation. Patients have proteinuria and microscopic hematuria and the proteinuria is predominantly non selective. Anti-inflammatory therapy with cyclophosphamide is effective treatment.

Focal and segmental glomerulonephritis: 

Here hyaline material is deposited in the subendothelial spaces of affected capillary in certain area thus called focal and segmental. Patients present with mild proteinuria or recurrent hematuria. The etiology is not known but is immune mediated. Low dose corticosteroid with addition of cyclosporine is used for treatment.

Classification of glomerulonephritis according to proteinuria:

Selective proteinuria:

It is a proteinuria in which the principal protein is albumin with relative paucity of Large Molecular Weight proteins like IgG.

Non-selective proteinuria: 

It is a proteinuria in which albumin along with large molecular weight proteins are found in urine almost equally.

Under normal circumstances glomerular barrier had a sharp molecular size cut-off above which protein molecules were excluded, but in patients with glomerular disease there is an increasing tendency to allow HMW proteins to pass into the tubules. Protein selectivity is based on a comparison of the relative concentrations of proteins of differing molecular weight in plasma and urine. Two proteins used for selectivity studies are IgG (150 kDa) and transferrin (69 kDa). Their relative clearances can be calculated as:

Clearance of IgG/Clearance of transferrin = [IgG]_u x [trans]_p/[IgG]_p x [trans]_u

Where [IgG]_u is urine IgG and [Trans]_p is plasma transferrin concentration.

Where the selectivity index is >0.5, the proteinuria is said to be non-selective, 0.3-0.5 moderately selective and <0.2 highly selective.

Patients with minimal change nephritis has highly selective proteinuria, membraneous glomerulonephritiss and proliferative glomerulonephritis show decreased selectivity and increased of HMW proteins relative to albumin. Patients with highly selective proteinuria tend to respond well to steroid therapy in contrast to those with non-selective proteinuria. Recently, coexisting tubular damage has been identified and monitored by measuring low molecular weight proteins like α₁-microglobulin or retinol binding protein (RBP) which are normally filtered by glomeruli and reabsorbed by renal tubules. Studies show progression to CRF is better predicted both by selectivity index or IgG excretion and tubular component of proteinuria (e.g. α₁-microglobulin) than by total 24 h urine protein excretion.

PROTEINURIA and it's types:

PROTEINURIA

Every 24 hour 65 kg protein goes to kidney and only 150 mg appear in urine. The glomerular barrier to filtration includes arterial fenestrated epithelium, glomerular basement membrane which is gel like structure, and overlying are epithelial cells called podocytes with numerous foot processes that interdigitated and envelop the outer surface of glomerular membrane. The foot processes are separated by slit diaphragms which form the final barrier to plasma proteins. The whole of the glomerular membrane carries net negative charge due to glycoproteins, sialic acid heparin sulfate with great density of charge at slit diaphragms.

The glomerular membrane selectively allows passage of water and low molecular weight solutes in the tubule and restricts passage of larger molecular weight plasma proteins. The negative charge of membrane restricts the passage of negatively charged plasma proteins forming electrochemical retention of plasma proteins (charge barriers especially starts in GBM and higher at slit diaphragm). Thus filtration of solutes depends upon their molecular size, shape and charge. Although some proteins are filtered e.g. albumin.

Most of proteins are reabsorbed in PCT and some are metabolized in tubular cell (e.g. 10-60% albumin). Only very small proteins with molecular weight <66 kDa (ie. Less than albumin mol. Wt.) normally reaches the urine (E.g. amylase, 48 kDa, myoglobin, 17.8 kDa, etc.).

Proteinuria occurs when filtered load increases and this may be due to glomerular damage, increased glomerular permeability, increased circulating concentration of LMW proteins or decrease in reabsorptive capacity due to tubular damage. There are 3 main types of proteinuria.

Glomerular:

Increased glomerular permeability. There is damage in basement membrane may be due to immune complex deposition, diabetic nephropathy. HMW proteins excretion is increased e.g. albumin, IgG.

Overflow: 

Increased plasma concentration of relatively freely filtered proteins. Bence Jones proteins in myeloma, lysozyme in leukaemia, amylase in pancreatitis, myoglobin in rhabdomyolysis, etc.

Tubular: 

Proximal tubular damage which leads to decreased reabsorption. This type of proteinuria also occurs due to decreased nephron number and there is increased filtered load per nephron. Proteins excreted are low molecular weight proteins like α₁-microglobulin, β₂-microglobulin, retinol binding protein, etc. Due to damage of tubular epithelial cells there is release of enzymes like ALP, N-acetyl-β-D-glucosaminidase). There is also distal tubular damage and during this proteins excreted are Tamm Horsfall glycoprotein, glutathione S-transferase (nephrogenic aminoaciduria)

Proteinuria can be detected by dipstick method which can detect >150 mg of protein in urine. 

Proteinuria >300 mg/day is pathological. Proteinuria above 1g/day implies glomerular proteinuria. Proteinuria has been shown to be a potential risk factor for progression of renal disease. It is also used for screening purpose to identify persons with silent kidney damage. There should be 50 to 60% loss of renal mass before appearing symptoms or alteration of biochemical markers.

There are at least 3 proteins present in urine that are renal origin they are Tamm Horsfall glycoprotein, urokinase and secretory IgA. THG is a glycoprotein and comprises 68% protein, 31% carbohydrate and 0.5 to 1.0% lipid. THG is secreted from the loops of Henle and the DCT. It traps albumin, red blood cells, tubular cells or cellular debris present in tubular fluid. It has protective role in trapping damaging material in urinary space, recently, the capacity of THG to bind E. coli has led to suggestion of its role for defense against UTI. Urokinase is a proteolytic enzyme that converts plasminogen to plasmin which in turn breakdown fibrin and has role in removing fibrin from the renal microvasculature and possibly in removing urinary casts. sIgA is produced by lymphocytes which is transported to tubular epithelial cells and secreted to lumen with secretory peptide which is a transport protein.

In the National kidney Foundation Guidelines, urine albumin is recommended as a sensitivity marker for chronic renal disease due to diabetes, glomerular disease and hypertension and α₁ and β₂-microglobulin as sensitive markers of tubulointerstitial disease.

Proteinuria is renal disease:

Proteinuria is the most frequent clinical finding in renal disease and quantitation of proteinuria is valuable test in monitoring renal damage. Urine protein is derived from plasma and from kidneys themselves. Measurement of low concentrations of specific proteins such as albumin, IgG or transferrin, predominantly reflecting glomerular function and α₁-microglobulin or retinol binding protein, reflecting tubular reabsorptive function, are now used as early markers of primary renal disease (e.g. glomerulonephritis) and secondary renal disease (e.g. due to DM or hypertension).

Conventionally proteinuria is classified into glomerular proteinuria, tubular proteinuria, nephrogenic proteinuria (e.g., due to THG, BM and tubular proteins), proteinuria of prerenal origin (e.g. overflow proteinuria like light chains disease, myoglobinuria, haemoglobinuria, lysozyme in leukaemia and amylase in pancreatitis) and postrenal proteinuria due to obstruction of the urinary tract or inflammation like in UTI.

DIABETES INSIPIDUS:

DIABETES INSIPIDUS:

There is disturbance in renal concentrating mechanism, resulting in polyuria nad inability to produce hypertonic urine. DI is due to absence of an ADH effect, either because of impaired or failed secretion (cranial or central DI) or lack of end organ response to ADH (nephrogenic DI). Most of the inherited nephrogenic DI have mutation in ADH V₂ receptor, there is also down regulation of AQP2 in acquired forms of DI.

Aminoacidurias: 

Amino acids are also entirely reabsorbed in PCT. There are 5 independent proximal tubular transport processes for amino acids, including those for

1.      Basic amino acids plus cysteine

2.      Glutamic and aspartic acid

3.      Neutral amino acids

4.      Imino acids and

5.      Glycine.  

Aminoacidurias can be overflow, glomerular, tubular and nephrogenic or secretory.

Cystinuria: 

It is a classic example of an amino aciduria due to true defect in renal tubular function. In most cases of cystinuria other dibasic amino acids like ornithine, arginine and lysine are also lost. The only known clinical manifestation of cystinuria is recurrent urinary calculi, the name cysteine coming from original (erroneous) assumption that the source of these stone was the bladder. Cystine stones form readily in acidic urine. 

They are yellow brown in color and are radio-opaque due to their sulphur content. These patients have higher incidence of calcium oxalate stones and all stone formers should be screened for cystinuria, preferably by formal amino acid measurement in the urine as qualitative screening test. This cystinuria is a metabolic disease and inheritance is autosomal recessive and individuals excrete cysteine, ornithine, arginine and lysine in urine. Defect or mutation in activator of amino acid transporter for cysteine and dibasic amino acids is found. Medical treatment of cystinuria begins with maintenance of high fluid intake throughout 24h and alkalinization of urine, both being aimed at decreasing the chance of precipitation of cysteine in the renal tract. Cystine solubilizing agent like D-penicillamine or chelating agent like captopril can be used. Otherwise cystinuria can damage kidney and result in CRF.

Cystine can be tested in urine by cynide-nitroprusside test, chromatographic techniques or other advanced techniques. In the cyanide-nitroprusside test, cysteine is split into two molecules of cysteine by cyanide. Sodium nitroprusside reacts with free sulfide groups to give a magenta color.

Hartnup disorder: 

In this condition there is also a true defect in renal tubular function. This is named after the family in which it was first described and is again a defect of both renal and intestinal amino acid transport. The feature is failure to reabsorb the neutral amino acids in renal tubule with their appearance in urine (E.g. Alanine, glycine, phenylalanine, tyrosine, asparagine, glutamine, etc). Most affected individual have increased amounts of indoles (e.g. indicant) in urine, which originates from the bacterial breakdown in the gut of unabsorbed tryptophan. There is pellagra like skin rash, renal amino aciduria and other features.

Dent’s disease:

It is X-linked nephrolithiasis, and is caused by mutation in gene coding for voltage gated chloride channel (CLC-5) expressed in PCT. The defect is thought to cause failure of the endosomal acidification that is required to allow recycling of the PCT membrane receptor megalin which mediates protein reabsorption. The syndrome is characterized by hypercalciuria, nephrocalcinosis, and LMW proteinuria. 

Inherited tubulopathies:

Inherited tubulopathies:

These disorders are characterized by electrolyte disturbances (particularly potassium).

Bartter’s syndrome: 

This is autosomal recessive characterized by renal salt wasting, polyuria, polydipsia, impaired urinary concentrating ability, hyperreninemic, hypokalemic metabolic alkalosis and mild hypomagnesemia. There is defect in three transporters/channels in loop of Henle, NKCC2 (type I) or ROMK1 (type II) and basolateral pump CLC-Kb (type III).

Gitelman’s syndrome: 

This is also autosomal recessive disorder characterized by hypokalemic, hyperreninemic, hypomagnesemic, metabolic alkalosis presenting latter in life than in batters syndrome. The molecular defect is in the thiazide sensitive NCCT transporter and biochemistry can therefore mimic the effects of thiazide use.

Liddle’s syndrome: 

This is autosomal dominant disorder characterized by hypokalemic, hypomagnesemic metabolic alkalosis but there is hypertension and hyperreninism. The disease is due to activating mutation which increases sodium transport through ENaC channel with increase in K and H loss.

Isolated abnormalities of tubular function:

Glycosuria: Glucose is freely filtered but reabsorbed completely in PCT. If plasma glucose concentrations rise or if GFR increases (as in pregnancy), then the capacity of PCT to reabsorb filtered glucose is exceeded and glycosuria occurs.

In hereditary glycosuria patients excrete glucose in urine at normal plasma glucose concentrations. There are two types of hereditary renal glycosuria (type A and B). The absorption of glucose in PCT is Na dependent by Na-D-glucose cotransport on luminal cell to transport glucose and by facilitated transport by GLUT2 on basolateral membrane which enable glucose to exit off the cell. In intestine there is Na-D-glucose cotransporter (SGLT1) and in kidney there is SGLT2. Mutation in gene for SGLT2 can cause hereditary renal glycosuria. Mutation affecting GLUT2 which facilitates transport of glucose, galactose and fructose across the basolateral membrane are another cause of renal glycosuria (Fanconi-Bickel syndrome).

TUBULAR DISEASE:

Renal tubular acidosis:

RTAs comprise disorders affecting either PCT or DCT. They are characterized by hyperchloremic, normal anion gap; metabolic acidosis. They are the result of either failure to retain bicarbonate or inability of renal tubules to secrete hydrogen ions.

The three categories of RTA are distal (dRTA, type I); proximal (pRTA, type II); and type IV, which is secondary to aldosterone deficiency or resistance. The term type III RTA (mixed proximal/distal defect) is not used.

Distal RTA (Type I): 

Clinical feature includes metabolic acidosis, muscle weakness, urolithiasis, nephrocalcinosis. The defect is an inability to secrete hydrogen ions in the distal tubule in the presence of systemic acidosis. Urinary pH >5.5 is a common feature. Due to inability to secrete hydrogen, Na reabsorption occurs by exchanging K in distal tubule and there is increased renal potassium loss producing hypokalaemia. The defect may be in H⁺-ATPase or H⁺, K⁺-ATPase transporter.

Proximal RTA (Type II): 

Here the primary defect is failure of proximal tubular bicarbonate reabsorption. Here the threshold of bicarbonate reclamation is lowered to 15 mmol/L from 22 mmol/L. The filtered load of bicarbonate when falls to a point at which hydrogen ion secretion is sufficient to reabsorb all filtered bicarbonate, then only bicarbonate is reclaimed. The plasma bicarbonate decreases to 15-20 mmol/L. Here the urinary pH will be <5.5 as the bicarbonate will be reclaimed after this threshold has reached.

Selective aldosterone deficiency (type IV RTA): 

There is failure of distal potassium and hydrogen ion secretion due to aldosterone deficiency or resistance. There is hyporeninemic hypoaldosteronism. Hyperkalemia is usual manifestation and failure to reabsorb sodium in exchange of chloride will produce hyperchloremia. There is also defective ammonia formation and may be due to glutaminase deficieny or defect.

Diagnosis of RTA:

The finding of early morning urinary pH <5.5 in presence of systemic acidosis supports the diagnosis of pRTA, since distal function is well. In dRTA the urinary pH is >5.5, hypophosphataemia.

The plasma potassium can give the clue (high in Type IV and low in Type I and II). In type IV there is also urine pH >5.5 but there is hyperkalaemia.

Fractional bicarbonate excretion can confirm diagnosis of pRTA as long as the patient’s plasma bicarbonate is maintained above 20 mmol/L

(Urine bicarbonate/plasma bicarbonate)/(urine creatinine/plasma creatinine) x 100%

In pRTA this is >10-15%, whereas in most cases of dRTA it is <10%.

Generalized tubular defects (Fanconi syndrome):

During this there is generalized renal tubular defect. There is a failure in net proximal tubular reabsorption of glucose, amino acids, phosphate and bicarbonate with consequent glycosuria, amino aciduria, phsophaturia and acidosis together with vitamin D-resistant metabolic bone disease. Although the exact mechanism is unknown; the cause may be inherited defect like cystinosis, fructose intolerance, galactosemia, glycogen storage disease type I and acquired like heavy metal poisoning, drugs (tetracycline, gentamicin), paraproteinaemia, amyloidosis. The clinical features are polyuria, polydipsia, dehydration, hypokalemia and acidosis, with impaired growth and rickets in children and osteomalacia in adults. 

Glomerular diseases:

GLOMERULAR DISEASE

These consists of rapidly progressive glomerulonephritis (RPGN), nephrotic syndrome, acute nephritis and chronic glomerulopathies.

Primary glomerular diseases present clinically with (1) abnormalities of the urine, including proteinuria and hematuria, (2) hypertension, (3) edema and (4) reduced renal excretory function.

Continuous production of protein free filtrate across a glomeruli by specialized mechanism for clearing trapped molecules through the mesangium increase the susceptibility by trapping circulating macromolecules. Deposition or generation of immune complexes can cause complement fixation and activation of lymphocytes and macrophages, all of which can mediate damage. Even environmental toxins can also induce glomerulonephritis in goodpasture’s syndrome and this might be from damage to BM lining of the lung which has some antigenic epitopes similar to those of glomerular BM.

Glomerulonephritis can be subdivided immunologically into conditions mediated by antibodies against either extrinsic or intrinsic to the kidney. Extrinsic agents include micro-organisms or DNA as in SLE, intrinsic components includes glomerular basement membrane, leading to immune complex formation. Whatever the mechanism is, antigen-antibody complexes become trapped in glomerulus, activating both the classical and alternative complement pathways and leading to release of anaphylatoxic components like C3a and C5a. Anaphylatoxins, together with locally released kinins, prostaglandins and leukotrienes, attract polymorphonuclear neutrophils to basement membrane where the latter releases lysososmal enzymes, leading to membrane disruption and glomerular proteinuria. There may be loss of anionic charge associated with epithelial foot process can produce massive selective proteinuria while changes in basement membrane, tend to be associated with increasingly non-selective proteinuria.

The primary glomerulonephritis are classified morphologically according to histological analysis into.

Rapidly progressive glomerulonephritis:

This can lead to kidney failure in only weeks or a few months. These are characterized by focal necrotizing glomerulonephritis. In response to fibrinogen and fibrin polymers that are released after glomerular capillary injury, proliferating epithelial cells and macrophages (leaking from circulation, as well as resident) eventually compress the glomerulus and obstruct the PCT compromising nephron function. RPGN can be idiopathic or secondary to other conditions like infection, autoimmune immune complex deposition like antineutrophil cytoplasmic antibodies (ANCA).

Minimal change glomerulonephritis: 

During this condition there is little or no abnormality and no immunoglobulin or complement components. Proteinuria involves loss of fixed negative charge on glomerular basement membrane due to fusion of epithelial cell foot processes. This leads to selective proteinuria (mainly albumin).

Membranous glomerulonephritis: 

It is the commonest cause of nephrotic syndrome in adults. There is thickening of glomerular basement membrane. There is IgG deposits seen most commonly. It may also be associated with SLE where immune complexes formed in circulation deposits in GBM. There is massive proteinuria and poorly selective and some have asymptomatic proteinuria or microscopic hematuria. Proposed hypothesis is the role of extrinsic antigen deposition in the glomerulus with subsequent IC formation, glomerular trapping of circulating IC or antibodies binding to intrinsic glomerular antigens.

Proliferative glomerulonephritis: 

Here glomeruli appear hypercellular, due, for example, to invading macrophages or mesangial cells. Different disease can elicit immunologically mediated glomerular damage, including bacterial endocarditis, Group A streptococcal infection and SLE. There is frequent IgA, IgG and C3 deposits, and complement activation. Patients have proteinuria and microscopic hematuria and the proteinuria is predominantly non selective.

Focal and segmental glomerulonephritis: 

Here hyaline material is deposited in the subendothelial spaces of affected capillary in certain area thus called focal and segmental. Patients present with mild proteinuria or recurrent hematuria. The etiology is not known but is immune mediated.

Nephrotic syndrome:

Gross changes in glomerular permeability characterize the nephrotic syndrome. The diagnostic criteria for establishing nephrotic syndrome are presence of proteinuria (>3g/day or albumin >1.5 g/day), hypoalbuminemia, hypercholesterolemia (elevated LDL, VLDL), lipiduria and finally edema. Edema occurs due to expansion of interstitial compartment and accumulation of sodium also there is loss of plasma oncotic pressure due to loss of protein (albumin). 

Proteinuria is a consequence of loss of charge selective properties of filtration barrier. Nephrotic syndrome can result from minimal change nephropathy; focal segmental glomerulosclerosis, membranous nephropathy associated with carcinoma, drugs, SLE and DN or may be idiopathic. Hematuria is not seen because damage is not large enough to allow RBC and oliguria is uncommon.

Acute nephritic syndrome:

This is characterized by rapid onset of hematuria, proteinuria (usually <3g/day), Azotemia, reduced GFR, and Na and water retention with resulting hypertension, oliguria and edema. This can be caused by PSGN, SLE, etc. Infiltration of glomerular mesangium or capillary with PMN leukocytes and monocytes can cause damage. 

There is also deposition of immune complex in basement membrane. Nephritic syndrome is the progression of nephrotic syndrome and hypercellularity and inflammation is more in this condition. 

Diabetic nephropathy and Causes of tissue damage by hyperglycemia:

Diabetic nephropathy:

Diabetes mellitus is a chronic hyperglycemia sufficient to cause multisystem dysfunction but mostly retina, kidney, nerves and arteries. Type 1 DM is due to autoimmune destruction of pancreatic islet beta cells causing loss of insulin. Type 2 diabetes is due to combination of cellular resistance to insulin and beta cell failure.

Diabetic nephropathy (DN) there is finding of proteinuria in patient with diabetes with no evidence of UTI. Overt nephropathy occurs when albumin excretion of around 300 mg/day. Albumin measurement in urine is the recommended for detecting and monitoring kidney damage in adults. Patients with albuminuria 30-300 mg/day are said to be having microabluminuria.

In DN, there is kidney necrosis, contraction, thickened basement membrane and other abnormalities. Clinical progression is defined in terms of change in urinary albumin excretion rate and decline in GFR and blood pressure. In type 1 DM, microabluminuria develops later after 5 years of diagnosis or at any time after 40 years of age. So annual monitoring or urine albumin is recommended after 5 years disease duration. Type 2 DM are usually older at presentation and defining the date of onset is difficult and annual monitoring of urine albumin is recommended from the time of diagnosis.

Hyperinfiltration with progressive increase in UAE with decrease in GFR can lead to hypertension and finally cause ESRD with uraemic syndrome.

To this date the best available predictor for development of nephropathy in kidney disease and in diabetics, is microabluminuria as UAE represents generalized vascular damage than renal microvascular injury alone.

The exact mechanism for hyperglycemic tissue damage probably includes

a.      Formation of Advanced glycation end products (AGE)

b.    Overactivity of polyol pathway. Polyols are sugar alcohols formed from their respective sugars under the action of aldose reductase.

c.       Generation of ROS

Glucose is preferentially shunted through the polyol pathway under hyperglycemic conditions generating sorbitol that accumulates within cells. Abundance of glucose and its product in ECM of glomerulus and interstitium is a key step in pathogenesis. There is excess of ECM within glomeruli and interstitium. E.g. TGF-β is stimulated by hyperglycemia, AGE, angiotensin II and ROS this TGF-β cause the upregulation of insulin independent GLUT-1 transporter in mesangial cells. Glucose is transported to cells through GLUT-1 and metabolized mainly be glycolytic pathway. Activation of other signaling pathways like PKC, DAG, MAPK, etc. induces expression of ECM proteins. Hypersclerotic and hypertrophic effect of glucose are largely mediated by TGFβ which is seen to be overproduced in DN patients.

The receptors for AGE has been identified (RAGE) which is selectively expressed in glomerular epithelial cells (podocytes). Increased accumulation of AGE in diabetes engages podocyte RAGE and may lead to increased glomerular permeability. Vascular permeability a hallmark of diabetes can be suppressed by inhibiting RAGE in animal model.

CHRONIC RENAL FAILURE:

CHRONIC RENAL FAILURE:

There is persistent renal impairment involving loss of both glomerular and tubular function. It gradually proceeds to ESRD. There is considerable decrease in number of functioning nephrons.  The GFR becomes less than 20 mL/min and there is uraemic syndrome. This condition is defined as either kidney damage or GFR<60 mL/min/1.73m² for at least 3 months. 

Classification of Chronic Kidney disease:

Stage	Description	eGFR (mL/min/1.73 m2)
1	Kidney damage with normal of increased GFR	=/>90
2	Kidney damage with mild decrease in GFR	60-89
3	Moderate decrease in GFR	30-59
4	Severe decrease in GFR	15-29
5	Kidney failure	<15 (or dialysis)

The common causes of CRF are glomerulonephritis, pyelonephritis, Diabetes mellitus, multisystem disease, etc.

In CRF most of nephrons are damaged, there is massive increase in renal blood flow due to decreased in afferent arteriolar resistance. This leads to increase in capillary hydrostatic pressure, increased permeability and passage of macromolecules, like proteins and lipoproteins through capillary wall. Some proteins are scavenged by mesangial cells, but overloading may cause functional derangement and cellular proliferation contributing glomerular sclerosis. Increased numbers of macrophages liberate growth factors that can damage endothelium and activate platelets and results in intraglomerular thrombosis with consequent fibrosis.

CRF causes hyperphosphataemia, and phosphate retention can contribute to renal damage through calcium phosphate precipitation. Dyslipidaemia is frequent finding and accumulation of lipids in glomeruli is thought to contribute to severity and progression of disease.

Developing kidney failure is most commonly monitored by changes in plasma creatinine concentration and calculated eGFR.

The Uraemic syndrome:

There is accumulation of uraemic toxins like urea (present in highest concentration), creatinine, hippuric acids, parathyroid hormone β₂-microglobulin, spermine, etc. in blood. It is more correctly called azotemia. Azotemia is the terminal manifestation of kidney failure. The biochemical characteristic of uremic syndrome includes:

·      

 Retained Nitrogenous metabolites like urea, creatinine, uric acids, Fluid, acid-base and electrolyte disturbances like metabolic acidosis, hyponatraemia, hypokalemia, hyperphosphatemia, hypocalcemia.

·         Abnormal lipid metabolism like hyper-TAG, decreased HDL, hyperlipoproteinemia, etc. 

Disturbances in CRF:

Retention of nitrogenous waste products: There is high plasma concentration of urea and creatinine. There is linear decrease in 1/Cr_p and thus GFR with time, in CRF. Hyperuricaemia is present but concentration does not often exceed 10 mg/dl and gout is rare.

Sodium and water metabolism: Sodium and water reabsorption is hampered causing sodium depletion and there is decreased ECF volume. Other solutes are also not reabsorbed nor secreted. Thus this lead to osmotic diuresis although due to reduced GFR there is no polyuria.

Potassium metabolism: Potassium balance can be maintained until the GFR falls below 5 mL/min. This is an adaptive response where distal tubular potassium secretion is increased; due to increased aldosterone secretion and increased sodium delivery to the distal tubule.

Kidneys excrete 40-80 mmol hydrogen ions per 24 h. In CRF this is impaired and there is acidosis as there is additional decrease in GFR. There is reduced phosphate excretion which diminishes the buffering capacity in urine, and reduced ammoniagenesis which also buffers hydrogen ions. There is defect in reclamation of filtered bicarbonate.

Calcium, phosphate and magnesium metabolism and renal osteodystrophy: CRF leads to decrease in calcitriol synthesis in kidney due to enzyme inhibition by retained phosphate and decrease amount of enzyme as renal mass decreases. Lack of calcitriol leads to decreased absorption of calcium from gut. This hypocalcaemia induce PTH production and mobilization of calcium from bone, in CRF PTH is very high in plasma but there is resistance to its action due to low calcitriol concentrations. So calcium levels are below normal.

Carbohydrate and lipid metabolism: There is impaired glucose tolerance may be due to insulin resistance. Dyslipoproteinaemia is present. There is increase in plasma Tg and VLDL, IDL. Total cholesterol and LDL are normal but HDL is reduced.