URINARY ALBUMIN EXCRETION (UAE) : INTRODUCTION AND ITS IMPLICATIONS

Patients with diabetes mellitus are at high risk of suffering renal damage. Diabetes is the most common cause of end -stage renal disease (ESRD). Although nephropathy is less common in patients with type 2 diabetes, approximately 60% of all cases of diabetic nephropathy occur in these patients because of the higher incidence of this form of diabetes. Early detection of diabetic nephropathy relies on tests of urinary excretion of albumin. Persistent proteinuria detectable by routine screening tests (equivalent to a urinary albumin excretion [UAE] rate greater than or equal to 30 mg/d) indicates overt diabetic nephropathy. Once diabetic nephropathy occurs, renal function deteriorates rapidly and renal insufficiency evolves. Treatment at this stage can retard the rate of progression but not stop or reverse the renal damage. Preceding this stage is a period of increased UAE not detected by routine methods. This range of 20 to 200 μg/min (or 30 to 300mg/24hr or albumin/creatinine ratio of 30-300 μg/mg) of increased UAE defines microalbuminuria. Note that it is not defined in terms of urinary albumin concentration, although the albumin: creatinine ratio can be used as a substitute for albumin measurements in a time collection of urine. The term microalbuminuria implies a small version of the albumin molecule rather than an excretion rate of albumin greater than normal but less than that detectable by routine methods. Clinical proteinuria or microalbuminuria is established with an albumin-creatinine ratio of ≥300 μg/mg or protein excretion ≥300 mg/day. 

The presence of increased UAE denotes an increase in the transcapillary escape rate of albumin and is therefore a marker of microvascular disease. Persistent UAE greater than 30 mg/d represents a twentyfold greater risk for the development of clinically overt renal disease in patients with type 1 and type 2 diabetes. Prospective studies have demonstrated that increased UAE precedes and is highly predictive of diabetic nephropathy, end-stage renal disease, cardiovascular mortality, and total mortality in patients with diabetes mellitus. In addition increase UAE identifies a group of nondiabetic subject at increased risk of coronary artery disease.

UAE is increased by physiological factors (e.g., exercise, posture, and diuresis) and the method of urine collection must be standardized. Samples should not be collected after exertion, in the presence of urinary tract infection, during acute illness, immediately after surgery, or after an acute fluid load. All the following urine samples are currently acceptable: 

(1) 24-hour collection; 

(2) overnight (8 to 12 hours, timed) collection; 

(3) 1- to 2-hour timed collection (in laboratory or Clinic); or 

(4) first morning sample for simultaneous albumin and creatinine measurement. 

Only results for timed specimens can be reported as mg albumin excreted per hour, but the albumin: creatinine ratio is more practical and convenient for the patient and is the recommended method. A first morning void sample is best because it has a lower within-person variation for the albumin: creatinine ratio than a random urine sample. At least three separate specimens, collected on different days, should be assayed because of the high intraindividual variation diurnal variation (50% to 100% higher during the day). Urine should be stored at 4⁰C after collection. Alternatively, 2 mL of 50 gm/L sodium azide can be added per 500 mL of urine, but preservatives are not recommended for some assays. Bacterial contamination and glucose have no effect. Specimens are stable for 2 weeks at 4 'C and for at least 5 months at -80⁰C. Albumin concentration decreased by 0.27% at -20⁰C. Freezing samples has been reported to decrease albumin, but mixing immediately before assay eliminates this effect.

The test strips most of which are optimized to read positive at predetermined albumin concentration have been recommended for screening programs. Test strips contains bromophenol blue in alkaline matrix to detect albumin concentrations exceeding 40 mg/L. Other test strips include antialbumin IgG complexed to galactosidase. The albumin in the urine binds to antibody enzyme conjugate in the test strip. Excess conjugate is retained in a separate zone containing immobilized albumin and only albumin bound to the antibody-enzyme immunocomplex diffuses to the reaction zone. Here it reacts with a buffered substrate (chlororphenol red galactoside) to produce a red color when the beta galactosidase hydrolyzes galactose.

For quantitation different RIA, ELISA radial immunodiffusion and immunoturbidimetry are available.

The ADA recommends initial UAE measurement in type 1 diabetes patients who have had diabetes more than or equal to 5 years and in all type 2 diabetic patients. Because of the difficulty in dating the onset of type 2 diabetes, screening should commence at diagnosis. Analysis should be performed annually in all patients who have a negative screening results. If screening result is positive UAE should be evaluated by quantitative assay. Diagnosis requires the demonstration of increased UAE in at least two of 3 tests measured within 6 month period.

(Source: Tietz Clinical Chemistry, 4th Edition)