THE RELIABILITY OF URINARY ALBUMIN ASSAY BY IMMUNONEPHELOMETRY IN THECLINICAL-PRACTICE - THE CRITICAL ROLE OF MEDICAL SURVEILLANCE ON LABORATORY ROUTINE

Measurement of the urinary albumin excretion rate (UAER) is essential for the early diagnosis and monitoring of diabetic nephropathy; immuno nephelometry is a procedure used worldwide for routine screening of di abetic patients. Since we have met with occasional inconsistent values of UAER in serial urine collections, we searched for possible sources of analytic error. To assess the best working conditions of the instr ument in use, the stability of urine samples during storage and the ne ed for previous urine centrifugation, we assayed repeatedly the six au tomatically diluted points of the standard curve (55.6 to 1.7 mg/l), f our control samples of human albumin in saline (100 to 1 mg/l) and 24- h urine collections from outpatient diabetic subjects. The last were a lso assayed with and without previous centrifugation, and both immedia tely after collection as well as after storage at -20 degrees C for 7, 42, 79, 97, 128 and 161 days. We concluded that: (1) pre-analytic cen trifugation of urine samples is unnecessary; (2) the intra-assay coeff icient of variation (CV) of the standard curve changed from 2.4% to 9. 3% when moving from the highest to the lowest concentration; the inter -assay CV changed from 4.1% to 14.4%, respectively; (3) the intraassay CV of the control samples (manually prepared) changed from 5.7% to 10 .2% and the inter-assay CV from 7.7% to 22.9%; there was a constant an d significant (P<0.01) underestimation (from -9% to -30%) of the obtai ned values compared with the expected concentrations; (4) a progressiv e decrease in recovered albumin by multiple freezing and thawing of ur ine samples did occur, which became significant after 161 days of stor age. In the BNA workbook (menu 7.1, assay protocols), a 7-day validity of the reference curve is reported. Moreover, to economize, pre-dilut ion cuvettes were often recycled in our hospital central laboratory. W e observed that: the intra-assay CV for urine samples was 79.4% with r ecycled cuvettes and stored standard curve, 11.3% with new cuvettes an d stored standard curve, 4.9% with both new cuvettes and newly perform ed standard curve; the inter-assay CV was 32.6%, 10.5% and 6.4%, respe ctively. These data emphasize, from the laboratory viewpoint, the need for both accurate calibration of BNA and use of native urines; in add ition, they stress the importance of careful supervision of laboratory routine and interpreting analytic results in the clinical setting.