Proteinuria is the excretion of protein in the urine. This condition is often an early warning sign of progressive kidney disease such as diabetic nephropathy. The loss of kidney function is one of several serious complications that can result from diabetes, particularly among those who have difficulty controlling their glucose (blood sugar) or blood pressure.

Proteinuria can also indicate high blood pressure and cardiovascular damage, which are also common complications of diabetes.

The kidneys are made up of millions of nephrons, which contain tiny clusters of blood vessels. These clusters, known as glomeruli, filter the blood and produce urine. Then the urine moves down into the collection ducts, through the ureters to the urinary bladder and through the urethra for expulsion from the body.

Although undetectable in normal urine tests, the beginning stages of proteinuria appear with the thickening of the glomeruli. In this early stage, the kidneys begin to let small amounts of albumin (the most common protein found in urine) pass through to the urine. Only highly sensitive tests can detect the presence of albumin at this stage, which is referred to as microalbuminuria.

Kidney disease normally progresses in three stages from microalbuminuria to proteinuria (larger quantities of protein in the urine) to end-stage renal disease (ESRD), in which the kidneys fail and dialysis or a kidney transplant is needed. Over the course of several years, diabetic nephropathy causes hyperfiltration, whereby increasingly large amounts of blood pass through the filters of the kidneys. Over time, the kidneys become worn down by this process. More and more protein begins to leak from the bloodstream into the urine, starting with microalbuminuria. If microalbuminuria goes undetected or untreated, proteinuria will result.                                 

With the progression of microalbuminuria to proteinuria, which can take as many as five to 10 years, more and more glomeruli become damaged. This, in turn, causes more essential protein to leave the body through the urine.

Once proteinuria has developed, enough protein is present in the urine to be detected through a normal urine test. If left untreated, proteinuria can lead to ESRD within two to six years.