(Micro)Albuminuria: Antigen Excess Detection in the Roche Modular Analyzer

Measurement of urinary albumin is performed both for detection of microalbuminuria (1) and for establishing the selectivity of urinary protein excretion when renal function deteriorates (2); it thus requires assays that can measure albumin concentrations from a few milligrams per liter to several grams per liter. Falsely low concentrations are encountered regularly in immunoturbidimetric assays as a result of antigen excess (3). Procedures to detect antigen excess include additional testing with qualitative measurement of urinary albumin by reagent strip (dipstick) (4)(5)(6), quantitative measurement of urinary total protein (7), or albumin analysis with multiple dilutions of the sample (8). Another procedure uses an excessive amount of antibody, which can lengthen the calibration curve considerably (9). All of these procedures are costly. Mathematical procedures based on the kinetic differences between samples with and without antigen excess have been proposed (9)(10), but they falsely suggest antigen excess in samples with low antigen concentrations. An efficient procedure involves adding additional albumin after the measurement is completed and interpreting the additional …