[Laboratory identification and measurement of urinary proteins]

A permanent and quantitatively abnormal proteinuria (> 150 mg/ 24 h) found at the laboratory should be followed by an identification and quantification of all proteins present. Specific immunochemistry techniques (immunoturbidimetric or immuno-nephelometric) are almost exclusively used for measuring albumin (or microalbuminuria) in nephropathic diabetic patients. Measurement of monoclonal free light chains is not recommended due to poor accuracy. Determination of other small proteins (molecular mass, MM < 40 kDa), transferrine, immunoglobulins G (assessment of selectivity) is far less frequent and Tamm-Horsfall protein is measured in specialized laboratories. Global electrophoresis techniques are the reference methods for the study of urinary proteins. Significant progress have been made during the past ten years in terms of sensitivity (analysis down to 50 mg/L total protein with no need to concentrate urinary specimens) and separation (high resolution or according to MM). Improvement in sensitivity is a key point for the diagnostic and follow-up of low grade or treated gammapathies. Electrophoretic separation according to MM is perfectly adapted to visual typing of renal proteinuria (glomerular, tubular or mixed) and overload proteinuria, particularly by monoclonal free light chains (MM: 25 kDa, Bence Jones proteinuria) allowing their quantification by densitometry. Immunofixation combining electrophoresis and immunoprecipitation in gel has emerged as the reference technique to identify monoclonal proteins. Data obtained by urinary protein analysis need to be compared with serum results for interpretation.