Proteinuria is a condition characterized by the presence of greater than normal amounts of protein in the urine. It is usually associated with some kind of disease or abnormality but may occasionally be seen in healthy individuals.
Plasma, the liquid portion of blood, contains many different proteins. One of the many functions of the kidneys is to conserve plasma protein so that it is not eliminated along with waste products when urine is produced. There are two mechanisms that normally prevent protein from passing into urine:
- The glomeruli provide a barrier that keeps most larger plasma proteins inside the blood vessels.
- The small proteins that do get through are almost entirely reabsorbed by the tubules.
Proteinuria most often occurs when either the glomeruli or tubules in the kidney are damaged. Inflammation and/or scarring of the glomeruli can allow increasing amounts of protein and sometimes red blood cells (RBCs) to leak into the urine. Damage to the tubules can prevent protein from being reabsorbed. Proteinuria may also develop when too much of a small protein is present in the blood and the tubules cannot reabsorb all of it.
Healthy people can have temporary or persistent proteinuria. It may be associated with stress, exercise, fever, aspirin therapy, and exposure to cold. Some people release more protein into the urine when they are standing up than when they are lying down (orthostatic proteinuria), though this condition is rare in individuals over age 30. However, a detectable level of protein in the urine usually indicates the presence of an underlying disease or condition and warrants further investigation to determine the cause.
The most common cause of protein in the urine is kidney damage resulting from:
- Diabetes – proteinuria is one of the first signs of deteriorating kidney function in people with type 1 and 2 diabetes.
- Hypertension – proteinuria in someone with high blood pressure is also a first sign of declining kidney function.
Others causes of kidney damage resulting in proteinuria include:
- Immune disorders (e.g., lupus, IgA nephropathy, Goodpasture's syndrome)
- Exposure to toxins
- Kidney cancer
- Congestive heart failure
Some other conditions that can cause proteinuria include:
- Red blood cell destruction and release of hemoglobin that occurs in the bloodstream (intravascular hemolysis)
- Pre-eclampsia – pregnant women are routinely screened for proteinuria because its presence is associated with pre-eclampsia (also known as toxemia of pregnancy). Pre-eclampsia is a pregnancy-specific disorder where proteinuria and hypertension develop at the same time. Symptoms can include edema (swelling), nausea, and headaches during pregnancy. Rarely, it can cause severe symptoms such as seizures. Pre-eclampsia can be dangerous for both the mother and her baby.
- Multiple myeloma (cancer of the plasma cells) – proteinuria due to the presence of excess proteins in blood that overflow into the urine (Bence-Jones protein) may be seen in multiple myeloma. Bence-Jones protein consists of an abnormal immunoglobulin light chain (either kappa or lambda) that is produced by monoclonal plasma cells. All immunoglobulins are composed of four protein chains, two light chains and two heavy chains. Bence-Jones protein is made of two immunoglobulin light chains and its presence in urine is often diagnostic of multiple myeloma in the context of other symptoms.