The presence of an increased amount of protein in a urine specimen is often the first indicator of renal disease. Proteinuria may signal severe kidney damage, be a warning of impending kidney involvement, or be transient and unrelated to the renal system. Further quantitative testing of urine for protein may be needed to determine the significance of the proteinuria.
Proteinuria related to kidney impairment may be due to glomerular membrane damage caused by toxic agents, immune complexes found in lupus erythematosus, or streptococcal glomerulonephritis. The amount of protein present in urine samples from patients with glomerular damage usually ranges from 10-40 mg/dL. If the urinary protein is due to a disorder that affects tubular reabsorption, the urine protein quantities will be much greater.
In patients with multiple myeloma, proteinuria is due to the excretion of the Bence Jones protein. This low molecular weight protein produced by a malignant clone of plasma cells circulates in the blood and is filtered in the kidneys in quantities exceeding the tubular capacity. This excess protein is excreted in the urine.