Proteinuria



In a normal individual, less than 150 mg of low molecular weight tubular protein is excreted in the urine per day. The glomerular basement mem­brane acts as an effective barrier to the passage of high molecular weight proteins, such as albumin, and the renal tubules function to reabsorb the small amount of protein that is filtered. Protein­uria may occur as a transient phenomenon in in­dividuals with febrile illnesses or congestive heart failure or after vigorous exercise. Persistent pro­teinuria, however, is almost always indicative of renal disease.

Nephrotic range proteinuria (>3.5 grams/24 hr) is indicative of glomerular disease. Lesser amounts of protein may be excreted in various tubulointerstitial diseases or in some cases of acute or chronic renal failure. When proteinuria is associated with red blood cell casts, glomer­ulonephritis should be suspected. Tubulointer­stitial pathology is suggested by the association of proteinuria with alterations in tubular function such as concentrating or acidifying defects. Pro­teinuria is often asymptomatic, although ne­phrotic range proteinuria will often result in hy-poalbuminemia and resultant edema formation.

The laboratory evaluation of the patient with proteinuria begins with a quantitation of protein excretion in a 24-hour urine specimen. A urine protein electrophoresis should be obtained to identify the major urinary proteins. Diseases of the glomerulus are characterized by predominant albuminuria, whereas tubulointerstitial disease results in the excretion of low molecular weight proteins. Detection of a monoclonal immunoglob­ulin spike in the urine is suggestive of multiple myeloma or amyloidosis. A renal biopsy should be considered when the cause of heavy protein­uria is not apparent from the initial evaluation.