[Biological analysis of proteinuria in the laboratory: quantitative features]

Total protein analysis is one of the most frequent laboratory analyses in urine. A proteinuria above 150 mg/L is often observed in a random way in preventive or school medicine (dipsticks) or during laboratory analysis (quantitative determination). Complete (quantitative, then qualitative) and repeated evaluation of proteinuria is of major interest for the clinician to establish a diagnosis of abnormality and for therapeutic follow-up of a nephropathy, uropathy or a non-renal disease (diabetes, multiple myeloma). Most frequent (90% of cases) and severe forms of proteinuria are of glomerular type, associated to the nephrotic syndrome, hypertension, and progressive renal failure. Attention should be paid by the biologist to the pre-analytical phase (specimen collection, treatment, and storage), to clinical data, and to prescription of drugs that could interfere with protein analysis. During the last past 10 years, significant analytical advances have been made: dipstick analysis has been dropped (false positives and most importantly false negatives) as manual precipitation techniques with turbidimetric detection (poor inter-laboratory coefficients of variation, CV), replacement of Coomassie blue by pyrogallol red (improved practicability). Urinary quality control data reflect these positive changes, as demonstrated by a dramatic reduction in reported CVs. There is, however, still no reference method for total urinary protein determination and limits of existing pyrogallol red methods should be emphasized: variable reagent composition between manufacturers (such as the presence of SDS additive), limited sensitivity, difficulty in the choice of a calibration material, underestimation of free light chains, and interference with gelatin based vascular replacement fluids.