The meaning of protein in the urine


January 18, 1994|By Dr. Simeon Margolis | Dr. Simeon Margolis,Contributing Writer

Q: Our company doctor told me during a routine physical examination that there was protein in my urine. I have never had any serious illnesses and feel perfectly well now. What is the significance of protein in the urine?

A: Protein in the urine (proteinuria) can be benign, but it is also an extremely sensitive marker of kidney disease. Factors that distinguish between benign and significant proteinuria include its persistence, the amount of protein in the urine and the presence or absence of other abnormalities in the urine. Some people have protein in their urine only when active (orthostatic or postural proteinuria); in these cases, the protein is not found in a urine specimen obtained after overnight sleep. Although the cause of orthostatic proteinuria is unknown, experience has shown that it goes away about half of the time and is usually not a warning sign of kidney disease. Exercise-related proteinuria is a variant of orthostatic proteinuria that occurs only following vigorous physical activity.

When proteinuria is detected, the urine is examined for red blood cells; their presence is another possible sign of kidney disease. The severity of the proteinuria is determined by measuring the protein in a 24-hour urine collection. Proteinuria of more than 0.5 grams of protein per day is evidence of kidney disease with the exception that some people with multiple myeloma, a form of malignancy, may produce large amounts of a protein that escapes into the urine.

Proteinuria can result from many types of kidney disease. The disease often originates in the kidney itself. The kidney may be damaged, however, during the course of a more generalized disease like diabetes, or from ingestion of a toxic substance, such as mercury, or an allergic reaction to a bee sting or a snake bite.

Individuals who consistently lose 3 or more grams of urine protein daily eventually develop a nephrotic syndrome, a characteristic set of changes resulting from the body's inability to cope with the loss of body protein. The components of the nephrotic syndrome are edema (swelling due to accumulation of fluid), low levels of the blood protein albumin (the major protein lost in the urine), and an elevated blood level of cholesterol and triglycerides.

Diabetes is one of the more common causes of proteinuria that advances to the nephrotic syndrome. The detection of even small amounts of urine (microalbuminuria) in a patient with diabetes is the first sign of kidney disease.

In others, the type of kidney disease responsible for the nephrotic syndrome, and for lesser degrees of proteinuria, can be determined reliably only with a biopsy of the kidney. About one fourth of adults with the nephrotic syndrome have lipoid nephrosis (also called nil disease or minimal change disease), which is associated with few changes in the biopsy and a course highlighted by periods of proteinuria interspersed with periods when protein is totally or nearly totally absent from the urine. Unlike the other kidney diseases that cause proteinuria, lipoid nephrosis generally does not lead to progressive deterioration of kidney function.

Dr. Margolis is professor of medicine and biological chemistry at the Johns Hopkins School of Medicine.

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