Exogenous and Endogenous markers for GFR:

A variety of exogenous (radioisotope and nonradioisotopic) and endogenous markers have been used to estimate clearance.

i.                    Exogenous markers:

These are recommended to measure GFR for monitoring slow progressing nephropathies such as that associated with diabetes. Both radioisotopic labeled and nonradioisotopic markers are used as exogenous markers. They are administered as constant infusion or as a single bolus.

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 Radioisotopic markers: These are ¹⁵Cr-EDTA, ^99mTc-diethylenetriaminepentaacetic acid (DTPA) and ¹²⁵I-iothalamate. These are of silver standard. The main disadvantage with them is time consuming, difficult testing, and risk of ionizing radiation. Among them ¹⁵Cr-EDTA is the preferred one as its clearance is close to inulin.

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  Nonradiosotopic markers: They include inulin (single bolus administration) and iohexol. These are of silver standard but continuous infusion of inulin is the gold standard.

Inulin clearance:

It is plant polysaccharide of fructose that satisfies all criteria (5 kDa). It is the gold standard for estimation of GFR. It involves constant injection of inulin for 3 hours at the rate of 18 mg/min and taking timed urine along with blood collection to measure inulin at the midpoints of the collection periods. The GFR is taken as the mean of inulin clearances for each period. Single bolus infusion of 70 mg/kg is done and blood and urine sample taken. Although due to lack of simplicity in testing, time consuming and difficult in performing it is not recommended for routine purpose.

Inulin clearance = 90-140 mL/min/1.73 m² in adults and 75-125 mL/min/1.73m² for >50 years of age and further declining with age.

ii.                  Endogenous markers of GFR:

Creatinine and certain low molecular weight proteins such as cystatin C have been used as endogenous markers of GFR. The use of urea is of limited value. These are Bronze standard tests.

Creatinine clearance:

It is the endogenous substance (113 Da), a normal product of muscle metabolism. It also satisfies all criteria except there is some active tubular secretion 7-10%. It is used as index of GFR as endogenous marker. For this also 24 hr urine is collected. The major cause of inaccuracy in determination of GFR by creatinine clearance is the inadequate collection and accurate measurement of urine volume of 24 hr. Measurement of creatinine coefficient as the ratio of mg of creatinine in 24 hr urine/body wt in kg. The value is 20-26 for males and 14-22 in females. This ratio is constant and serves as a reliable index of adequacy of 24 hr urine collection.

The methodological inaccuracies are also another factor for error as creatinine measurement has many interferences e.g. by glucose, ketones, bilirubin, hemoglobin, ascorbate, etc.

The plasma creatinine is inversely related to GFR. Halving the GFR will double the plasma creatinine concentration. During early renal disease the plasma creatinine is within normal range thus it is relatively insensitive index of mild renal dysfunction. Other factors contributing error are diet (recent non veg diet can cause transient increase in creatinine), muscle mass (high muscle mass high creatinine), age (decline in GFR with age and decrease in muscle mass with age and thus decrease in creatinine concentration), presence of interfering substance like ketones, bilirubin, cephalosporin, high glucose, etc.

Creatinine clearance = 93 – 130 mL/min/1.73m² for adult males and 70-125 for more than >50 years male, whereas 60-110 for females >50 years of age. This means that amount of creatinine excreted in urine in one minute is equal to amount found in 1.1 to 1.3L/min or 750 ml plasma/min.

Cystatin C:

This is LMW protein (12.8 kDa, pI 9.2), and a cysteine protease inhibitor present on the surface of all nucleated cells and shed into plasma and excreted by glomerular filtration. It is more stable than creatinine and has no affecting variable like age, height, muscularity, etc. Studies suggest it to be more sensitive than creatinine in assessing GFR even in mild renal impairment. But its measurement is quite difficult and expensive