Proteinuria is a condition that occurs when there is a greater than normal amount of protein in the urine. It is usually associated with some kind of underlying disease or abnormality but may occasionally be seen in healthy individuals.
Urine normally contains a small amount of protein. The kidneys, which are part of the urinary tract, filter wastes from the blood and produce urine. The liquid portion of blood (plasma) contains many different proteins and one of the many functions of the kidneys is to prevent plasma protein from being eliminated along with waste products when urine is produced. There are two mechanisms that normally prevent protein from passing into urine:
- Within the kidneys are about a million tiny blood filtering units called nephrons. In each nephron, blood is continually filtered through a cluster of looping blood vessels, called a glomerulus (plural is glomeruli). The glomeruli provide a barrier that keeps most larger plasma proteins inside the blood vessels.
- Attached to each glomerulus are tubes (tubules) that have a number of sections that collect the fluid and molecules that pass through the glomerulus. The small proteins that do get through the glomerulus 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.
There are conditions that can cause healthy people to have temporary (or transient) or persistent proteinuria but don't necessarily indicate kidney damage. Examples include dehydration, stress, strenuous exercise, fever, aspirin therapy, and/or exposure to extreme cold.
Some people release more protein into their 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, too much 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 people with high blood pressure is also a first sign of declining kidney function.
Some examples of other causes of kidney damage resulting in proteinuria include:
- Amyloidosis (buildup of abnormal proteins in organs)
- Kidney disease (e.g., chronic kidney disease, glomerulonephritis)
- Medications (e.g., non-steroidal anti-inflammatory drugs, other pain killers)
- Immune disorders (e.g., lupus, IgA nephropathy, Goodpasture syndrome, rheumatoid arthritis)
- Infections (e.g., endocarditis, kidney)
- Exposure to toxins, such as heavy metals and ethylene glycol
- Trauma, injury
- Cancer (e.g., Hodgkin lymphoma, kidney cancer, multiple myeloma)
- Heart disease, congestive heart failure
Some additional conditions that can cause proteinuria include:
- Red blood cell destruction and release of hemoglobin that occurs in the bloodstream (intravascular hemolysis)—this can occur with hemolytic anemia, for example.
- Preeclampsia – 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.