Patient information: Protein in the urine (proteinuria)

OVERVIEW

The kidneys primary function 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 filtering units in the kidney called nephrons (figure 2). Each nephron is composed of a glomerulus and a tubule. Each kidney contains thousands of nephrons.

The glomeruli filter the blood, removing waste products from the blood stream. They also function to prevent some substances, such as protein, from being filtered out of the blood. If the glomeruli are damaged as a result of inflammation (called glomerulonephritis) or an underlying disease, protein in the blood is abnormally filtered into the urine. This leads to the loss of protein through the urine. Abnormal amounts of protein in the urine can also develop when abnormal proteins are filtered from the blood, or if the tubules do not absorb the filtered protein.

Normally, the kidneys should excrete less than 150 milligrams of protein in the urine per day. Protein excretion above 150 milligrams per day is called proteinuria.

DOES PROTEINURIA CAUSE SYMPTOMS?

Most patients 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 you excrete 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. It is not known why orthostatic proteinuria occurs.

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 'Estimating protein excretion' 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 a bodywide disorder. Examples of these include the following:

• Heart or blood vessel disease
• A disease in the body that produces abnormal proteins
• A disorder that affects the glomeruli or tubule function (eg, kidney disease).

The amount of protein found in the urine in proteinuria can vary widely, from a fraction of a gram to over 20 grams (20,000 milligrams) per day. As mentioned above, proteinuria is defined as protein excretion greater than 150 milligrams per day

PROTEINURIA DIAGNOSIS

Proteinuria is detected by analyzing the urine (urinalysis), often with a dipstick test. However, dipstick testing is not very precise, and further testing is needed to confirm how much protein you excrete.

Repeat measurement — Since certain disorders or conditions (such as fever) may temporarily increase the amount of protein in the urine, testing is usually repeated. Transient proteinuria is common and repeat urine tests are usually normal.

Microscopic urinalysis — The urine is also examined with a microscope to see if there are cells, crystals, bacteria, or structures called casts. (See "Patient information: Glomerular disease overview".)

History and physical examination — If the initial urinalysis is positive for protein, a healthcare provider will perform a complete medical history and physical examination.

Estimating protein excretion — If two or more urinalyses are positive for protein, the next step is to determine the rate of protein excretion. This can be measured from a random urine specimen (this is the most common method) or from urine that has been collected over 24 hours. The rate is usually calculated from the total protein-to-creatinine ratio. Creatinine is another substance normally found in the urine. (See "Patient information: Collection of a 24-hour urine specimen".)

Blood testing may also be recommended to measure kidney function; this includes measurement of BUN (blood urea nitrogen) and plasma creatinine concentration and estimation of glomerular function (glomerular filtration rate) with a formula.

Nephrology evaluation — For patients who require further evaluation, a nephrologist may recommend further urine and blood testing and a kidney ultrasound. The ultrasound measures the kidneys and can determine if there are any abnormal structures.

Renal biopsy — A renal biopsy is performed to establish the diagnosis of the underlying kidney disease in a patient with persistent mild proteinuria (less than 2 grams/day) if there is a sign of progressive disease, such as increasing protein excretion, high blood pressure, abnormal microscopic urinalysis, or evidence of reduced kidney function. (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. Because no two people are exactly alike and recommendations can vary from one person to another, it is important to seek guidance from a provider who is familiar with your individual situation.

This discussion will be updated as needed every four months on our web site (www.uptodate.com/patients). Additional topics as well as selected discussions written for healthcare professionals are also available for those who would like more detailed information.

Some of the most pertinent include:

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 pregnant women
Measurement of urinary protein excretion
Orthostatic or postural proteinuria
Overview of heavy proteinuria and the nephrotic syndrome

A number of web sites have information about medical problems and treatments, although it can be difficult to know which sites are reputable. Information provided by the National Institutes of Health, national medical societies and some other well-established organizations are often reliable sources of information, although the frequency with which they are updated is variable.

• 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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