OVERVIEW

The kidneys job is to remove wastes and excess water and salts from the blood. Kidneys receive blood through the renal arteries (figure 1). The blood flows into parts of the kidney called nephrons (figure 2). Each nephron is made of a glomerulus and a tubule. Each kidney contains thousands of nephrons.

The glomeruli filter the blood, removing waste products from the blood. They also prevent some substances, such as protein, from being taken out of the blood. If the glomeruli are damaged, protein from the blood leaks into the urine.

Normally, you should have less than 150 milligrams of protein in the urine per day. Having more than 150 milligrams per day is called proteinuria.

DOES PROTEINURIA CAUSE SYMPTOMS?

Most people with proteinuria have no signs or symptoms. However, some patients have edema (swelling) in the face, legs, or both.

TYPES OF PROTEINURIA

Proteinuria can be divided into three categories: transient (intermittent), orthostatic (related to sitting/standing or lying down), and persistent (always present).

Transient proteinuria — Transient (intermittent) proteinuria is by far the most common form of proteinuria. Transient proteinuria usually resolves without treatment. Stresses such as fever and exercise may cause transient proteinuria.

Orthostatic proteinuria — Orthostatic proteinuria occurs when your kidneys filter out a normal amount of protein when lying down but an increased amount when you are sitting or standing. It occurs in 2 to 5 percent of adolescents, but is unusual in people over the age of 30. The cause of orthostatic proteinuria is not known.

Orthostatic proteinuria is diagnosed by obtaining a split urine collection. This requires collecting two urine sample: one while you are standing or sitting up (usually during the day) and another after you have been sleeping for several hours (eg, first thing in the morning) (see 'Urine testing' below). Orthostatic proteinuria is not harmful, does not require treatment, and typically disappears with age.

Persistent proteinuria — In contrast to transient and orthostatic proteinuria, persistent proteinuria occurs in people with underlying kidney disease or other medical problem. Examples include:

• Heart or blood vessel disease
• A disease in the body that produces abnormal proteins
• A disorder that affects how the kidneys work (eg, kidney disease).

PROTEINURIA DIAGNOSIS

Urine testing — Proteinuria is diagnosed by analyzing the urine (called a urinalysis), often with a dipstick test. However, dipstick testing is not very precise. Most people need to have the urine test repeated. It is common to have proteinuria temporarily, and repeat urine test are usually normal.

The urine will also be examined with a microscope to see if there are cells, crystals, bacteria, or structures called casts. These things can be a sign of another kidney problem, called glomerular disease. (See "Patient information: Glomerular disease overview".)

If two or more urinalyses show protein in the urine, the next step is to determine how much protein is in the urine. This can be measured from:

• A single urine sample collected at any time (this is the most common method)
• Urine that has been collected over 24 hours. (See "Patient information: Collection of a 24-hour urine specimen".)

If your doctor or nurse asks you to collect urine at home, try to keep it in a cool place, like the refrigerator.

Blood testing — Your doctor or nurse might also ask you to have blood tests to see how well your kidneys are working (called kidney function testing). This includes measurement of BUN (blood urea nitrogen), creatinine, and then calculating how well the kidneys work with a formula called glomerular filtration rate.

Renal biopsy — Your doctor might recommend a test called a renal biopsy. During a biopsy, a doctor takes a small piece of one kidney and then looks at the tissue under the microscope. Most people with proteinuria will not need a kidney biopsy. (See "Patient information: Renal (kidney) biopsy".)

PROTEINURIA TREATMENT AND PROGNOSIS

Transient and orthostatic proteinuria are not harmful conditions and no treatment is needed.

Patients with persistent low-grade proteinuria that is not related to decreased kidney function or a systemic disease typically have no long-term complications, even if untreated. Many nephrologists use an antihypertensive drug, such as an angiotensin converting enzyme (ACE) inhibitor, to reduce or eliminate proteinuria.

In patients with persistent high-grade proteinuria who have decreased kidney function, the underlying condition is usually treated. (See "Patient information: Chronic kidney disease".)

SUMMARY

• Normally, protein should not be found in the urine. A person who has protein in the urine is said to have proteinuria.
• Most people have no signs or symptoms of proteinuria.
• Proteinuria is usually discovered with a urine dipstick test that is done for another reason.
• There are three types of proteinuria: transient (temporary), orthostatic (related to sitting/standing or lying down), and persistent (always present).
• Several urine tests are needed to determine which type of proteinuria a person has. Some people also need blood tests or an exam by a doctor or nurse.
• Transient and orthostatic proteinuria do not cause long-lasting health problems and do not usually need to be treated.
• Some people with persistent proteinuria have kidney problems that need to be treated.

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem.

This article will be updated as needed every four months on our web site (www.uptodate.com/patients).

Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

Patient level information

Patient information: Glomerular disease overview
Patient information: Collection of a 24-hour urine specimen
Patient information: Renal (kidney) biopsy
Patient information: Chronic kidney disease

Professional level information

Evaluation of isolated proteinuria in adults
Evaluation of proteinuria in pregnancy
Measurement of urinary protein excretion
Orthostatic or postural proteinuria
Overview of heavy proteinuria and the nephrotic syndrome

The following organizations also provide reliable health information.

• National Library of Medicine

      (www.nlm.nih.gov/medlineplus/healthtopics.html)

• National Institute of Diabetes and Digestive and Kidney Diseases

      (www.niddk.nih.gov)

• National Kidney Foundation

      (www.kidney.org)

• American Kidney Fund

      (www.kidneyfund.org)

• American Association of Kidney Patients

      (www.aakp.org)

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