Relevance of urinary specific protein assay in the diagnosis of kidney diseases.

The analysis of urinary protein composition is an important step in the evaluation and monitoring of kidney diseases. Among the various approaches, the determination of urinary-specific proteins makes it possible to non-invasively detect a preferentially tubular or glomerular injury, to orientate towards a pathological process, to guide the indication of a kidney biopsy, and to follow the evolution of the disease and the effectiveness of a therapy. No study systematically evaluated the performance of urinary-specific proteins for the diagnosis of a renal disease. We conducted this retrospective study to perform an exhaustive analysis of the correlations that may exist between histologically proven kidney disease and the corresponding specific urinary protein composition it in order to evaluate the diagnostic value of each of its components. Urinary concentrations of total protein, albumin, transferrin, alpha1microglobulin, beta2microglobulin, retinol binding protein, and immunoglobulin G were analyzed in more than 500 patients who underwent renal biopsy and concomitant urine specific protein analysis. Our analysis shows that these markers have a limited positive predictive value in this cohort of complex and unselected kidney diseases. In particular, low molecular weight proteins, and especially alpha1microglobulin, are frequently associated with glomerular diseases. We identified transferrin as an independent predictor of minimal changes disese and renal amyloidosis, and beta2microglobulin as an independent predictor of acute tubulointerstitial nephropathy and myelomatous tubulopathy. Finally, we defined the thresholds at which these parameters had excellent negative predictive values.