Overview of Proteinuria
Proteinuria is an abnormally high amount of protein in the urine. Proteins in the blood, like albumin and immunoglobulin, help coagulation (clotting), balance bodily fluids, and fight infection. The kidneys remove wastes from protein-rich blood through millions of tiny filtering screens called glomeruli.
Most proteins are too large to pass through the glomeruli into the urine. The glomeruli are negatively charged, so they repel the negatively charged proteins. Thus, a size and charge barrier keeps protein molecules from entering the urine. But when the glomeruli are damaged, proteins of various sizes pass through them and are excreted in the urine.
Types of Proteinuria
The following five types of proteinuria are distinguished by milligrams (mg) of protein measured during a 24-hour urine collection:
- Microalbuminuria (30150 mg)
- Mild (150500 mg)
- Moderate (5001000 mg)
- Heavy (10003000 mg)
- Nephrotic range (more than 3500 mg)
As kidney disease progresses, more protein enters the urine. People with nephrotic-range proteinuria typically have extensive glomeruli damage and usually develop nephrotic syndrome.
Causes and Risk Factors for Proteinuria
The following conditions cause proteinuria:
- Acute glomerulonephritis
- Amyloidosis (protein deposits associated with chronic disease)
- Focal glomerulonephritis
- IgA nephropathy
- Mesangial proliferation
- Minimal change disease
Proteinuia Signs and Symptoms
Foamy urine and swelling (edema) are two signs of proteinuria that become more evident as the disease progresses. Excess protein can cause the urine to foam in water. This occurs because protein changes the surface tension between urine and water. Edema usually only occurs in nephrotic range proteinuria.
Albumin is particularly useful in absorbing bodily fluid into the blood. Because the albumin molecule is relatively small, it is often among the first proteins to enter the urine after glomeruli are damaged. Therefore, even minor kidney dysfunction is detectable with proper diagnosis of micoralbuminuria. Reduced albumin level in the blood causes fluid retention and swelling that is first noticeable in the hands, lower legs, and feet. In more serious cases, the abdomen and face may swell.
Orthostatic proteinuria is a disorder seen occasionally in children and young adults who leak significant amounts of urine when they are upright (orthostatic). Presumably, standing increases the pressure on the glomeruli and causes more protein to enter the urine, while lying down relieves pressure and causes less protein leakage. This is a benign disorder that most young people outgrow.
Hypertensive people who develop proteinuria stand a significant chance for kidney failure. African Americans are 20 times more likely than Caucasians to develop hypertensive-related kidney failure. Proteinuria in people with diabetes may be a sign that kidney disease is worsening. Microalbuminuria is often cited as a risk for coronary artery disease (CAD) and is often diagnostic of it and related cardiovascular conditions.