Proteinuria is the presence of protein in the urine, which is often an early indication of the kidney disease diabetic nephropathy. Diabetic nephropathy is the leading cause of chronic kidney failure in the United States.

When the kidneys are functioning properly, arteries bring blood and waste from the bloodstream into the kidneys. Tiny filters in the kidneys called glomeruli clean the blood and then send out the waste through the bladder to be eliminated as urine. The clean blood leaves the kidneys through veins and returns to the bloodstream.

During the blood-filtering process, glomeruli in healthy kidneys keep protein inside the body. Most proteins are too large to get past the glomeruli and into the urine, unless the glomeruli become damaged. Diabetes and high blood pressure are the main causes of proteinuria. Other sources include infections, preeclampsia, certain medications and some heart conditions.

Over a period of years, hyperglycemia (high blood glucose) and high blood pressure can cause significant damage to the glomeruli. Once these filters are damaged, protein can pass through and leak into the urine. The most common protein to appear in urine is albumin because it is small and can pass more easily through the glomeruli. The main function of this particular protein is to retain fluid in the blood.

When the kidneys’ filters become damaged, waste and excess fluids stay inside the body instead of being eliminated in urine. These waste products and extra fluids build up in the blood, while small amounts of protein leak out through urine.

When the amount of protein present in the urine is small, it is called microalbuminuria. The leakage of protein in the urine can be present for years and even become progressively worse with no signs or symptoms. As the damage worsens, the kidneys lose more and more protein, resulting in proteinuria. Often the damage becomes so severe that the kidneys ultimately fail, necessitating dialysis or kidney transplant.