There are over a hundred different types of proteins in the blood and the kidneys are very good at keeping them from entering the urine. Most of the protein that does make it into the urine are reabsorbed, chewed up and returned to the blood. As a result, less than 150 mg of protein is normal lost in the urine per day. A higher level of protein loss in the urine is called
proteinuria and may mean there is kidney disease.
Types of Urine Protein
The most important urine proteins found with kidney disease are
albumin and
antibody fragments. Albumin is the most abundant protein in the blood and it is important in transporting other things around the body as well as keeping fluid from leaking out into the tissues. Antibody fragments result from diseases involving the cells that produce antibodies (such as multiple myeloma). When the kidneys are unable to absorb all of them, they spill over into the urine.
The easiest method to screen for protein is the
urine dipstick. It is a good at detecting a large amount of protein the urine, but not as good for smaller amounts (microalbuminuria). In certain people, particularly those with diabetes, it is important to know when there is even a small amount of protein or albumin in urine. In this case, you need to measure the "urine albumin" directly.
Once protein is found in the urine, it is often important to measure how much is protein is lost in a day. Collecting urine for a 24-hour period (
24-hour urine collection) and then measuring all the protein in it been considered the gold standard. However, because it is very inconvenient and prone to error if the urine isn’t collected properly, we usually use the urine protein (or albumin) to creatinine ratio instead. It is requires only a small sample of urine and it is relatively accurate and convenient.
Transient Protein in the Urine
Transient proteinuria is the most common cause of proteinuria and been estimated to affect between 4-7% of people. It is a temporary process that is thought to be caused by fever, heavy exercise, and other benign conditions. The amount of protein in the urine is relatively small and usually resolves pretty quickly.
Persistent Protein in the Urine
Proteinuria that is constant or persistent is more likely to be a problem and may require a workup. It is usually caused by diseases involving the
glomerulus, the filtering device of the kidneys. The most common cause is diabetes mellitus but it many other diseases, conditions, and even medications can cause it.
Working Up of Protein in Urine
Unless there is a lot of protein in the urine, the first thing to do is determine if it is persistent (found on three occasions) or not. If it is, a workup is usually started which includes quantifying the amount of protein in the urine, performing a careful medical history and physical exam, looking at the urine under a microscope, and obtaining various laboratory tests. In some cases, particularly if there is severe protein loss or signs of kidney failure, a kidney biopsy may be necessary.
Treating Protein in the Urine
Specific treatment depends on diagnosis and amount of protein being lost. Most everyone should be treated with an ACE inhibitor (or angiotensin blocker) and good blood pressure control. Many people with low-grade protein will receive no additional treatment. Some people, especially those with autoimmune or inflammatory diseases, may be started on powerful drugs affecting their immune system. Again, the cause of the proteinuria is key to choosing a particular treatment.
Understanding the Complications
High levels of protein loss can lead to a condition called nephrotic syndrome, which typically causes a lot of problems with fluid retention and swelling (
edema) as well cholesterol and lipid problems (
hyperlipidemia). If it is severe enough, you may develop blood clots, infections or malnutrition.
Knowing the Prognosis
The prognosis depends on diagnosis and amount of protein being lost. In general, the larger the protein loss, the worse the prognosis. Many people with no obvious cause and relatively small amounts of protein loss (less than 1-2 grams of protein) do reasonably well without treatment. Unfortunately, there are also a number of serious kidney diseases that can lead to kidney failure.
