What is Proteinuria?

The function of your kidneys is to filter waste products from your blood and remove them from your body through the formation of urine. The functional unit of the kidney – the nephron – regulates the removal and reabsorption of chemicals and water to ensure the proper levels of salts and minerals for normal body function.

In a healthy kidney, a small amount of protein passes through the glomerular filtration barrier. Most of the protein is reabsorbed in the proximal tubules, so normally only small amounts of protein are present in urine (< 15 mg/dL). Temporarily increased amounts of protein may be due to exercise, dehydration or inflammation; however, In certain diseases or defects, the kidneys may allow excessive protein to pass through the glomeruli or may not be able to reabsorb the protein. This results in sustained elevated levels of protein in urine, or proteinuria.

Protein is one of the parameters measured on a standard urinalysis (UA) dipstick, and testing can be performed manually or by automated methods. Proteinuria is often the first indicator of kidney disease, although, increased protein is not present in all renal disorders nor is renal disease the only cause of proteinuria. For this reason, prolonged elevated levels of protein in urine require additional testing to determine the cause, which can generally be classified as prerenal, renal or postrenal.

Prerenal proteinuria is caused by increased plasma proteins that overflow across the glomerular barrier. These proteins may be:

• Acute phase reactants seen in many conditions including septicemia.
• Hemoglobin or myoglobin.
• Kappa or lambda monoclonal light chains that are increased in multiple myeloma and macroglobulinemia (historically termed Bence Jones proteins).

Renal proteinuria can be due to glomerular or tubular defects. There are many conditions that can be associated with renal proteinuria, including systemic and infectious diseases, drug toxicity and renal disease. Patients with confirmed kidney disease may be asked to test their urine protein levels at home using dipstick kits to monitor ongoing or increased damage to their kidneys.

Postrenal proteinuria is likely due to inflammation, malignancy or injury in the urinary tract.

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Resources:

Fundamentals of Urine and Body Fluid Analysis, Nancy A Brunzel, W. B. Saunders Co., Philadelphia, PA