Protein in urine: what it means and what happens next

A urine test report may say "protein +", "proteinuria", "albumin detected" or give an ACR number. It can be unsettling to read. Protein in urine is common, it can be temporary, and one result is not a diagnosis. But when it is confirmed and persists, it is one of the most important early signs of kidney damage, so it deserves proper follow-up.

What protein in urine means

The kidneys contain about a million tiny filters, called glomeruli. They let water and waste pass from the blood into the urine, but they normally hold back larger molecules such as proteins. The main protein in blood is albumin.

When the filters are damaged or under strain, albumin and other proteins can pass into the urine. Doctors call this:

• Proteinuria: protein of any kind in the urine.
• Albuminuria: albumin in the urine. This is the measure KDIGO uses to assess kidney damage.

Protein in urine usually causes no symptoms. Large amounts can make urine foamy and cause swelling, but most people only find out from a test.

How protein in urine is measured

• Urine dipstick. A quick strip that changes colour. Results are shown as negative, trace, +, ++ or +++. It is a useful first check but not precise, and it can be affected by very concentrated or very dilute urine.
• Urine albumin-to-creatinine ratio (ACR). A lab test on a single urine sample that compares albumin with creatinine, which corrects for how concentrated the urine is. KDIGO recommends ACR as the preferred test, ideally on an early morning sample. See urine ACR test.
• Urine protein-to-creatinine ratio (PCR). Measures all proteins. Some clinicians use it, particularly when protein levels are high.
• 24-hour urine collection. Collecting all urine for a full day. It is used less often now because ACR and PCR are easier.

KDIGO groups ACR results into three categories. Your lab may report in mg/mmol or mg/g, so check the unit on your report.

Temporary causes

Protein can appear in urine for a short time without any lasting kidney problem. A clinician may consider:

• fever or an acute illness;
• a urinary tract infection;
• very hard exercise in the day before the test;
• menstruation, which can contaminate the sample;
• very high blood pressure or very high blood sugar at the time;
• in some young people, protein that appears only after standing for a long time and disappears when lying down (orthostatic proteinuria), which is usually harmless.

For these reasons a raised result is usually repeated, often on an early morning sample, before a clinician decides what it means.

Causes of persistent protein in urine

If protein is still there on repeat tests, a clinician will look for the reason. Common ones include:

• Diabetes. Over time, high blood sugar can damage the kidney filters. Albuminuria is often one of the first signs. See diabetes and kidney health.
• High blood pressure, which puts strain on the filters. See high blood pressure and kidneys.
• Glomerular diseases, a group of conditions where the filters become inflamed or damaged, sometimes linked to infections or immune conditions.
• Other kidney conditions, including inherited ones.
• Pregnancy-related conditions. New protein in the urine with high blood pressure during pregnancy can be a sign of pre-eclampsia, which needs prompt assessment.

Why it matters

KDIGO 2024 describes chronic kidney disease (CKD) as abnormalities of kidney structure or function present for more than 3 months. An ACR of 3 mg/mmol (30 mg/g) or more that persists beyond 3 months is one of these markers, even when eGFR is normal. So protein in urine can show kidney damage before creatinine rises.

Clinicians look at albuminuria together with eGFR. The combination of the two gives a better picture of risk than either alone: higher albumin levels are linked with a higher chance of kidney function falling over time, and with heart and blood vessel disease. The good news is that finding albuminuria early gives time to act. A clinician may discuss blood pressure and blood sugar targets, lifestyle changes, and medicines that can protect the kidneys. Those decisions depend on the individual and are made with your doctor.

What usually happens next

• A repeat urine ACR to confirm the result.
• A blood test for creatinine and eGFR. See eGFR explained.
• A urinalysis to check for blood or infection. See urinalysis.
• Blood pressure and blood sugar checks.
• Sometimes a kidney ultrasound.

Referral to a nephrologist may be considered when a lot of albumin is found, when protein comes with blood in the urine, or when eGFR is low or falling. Read when to see a nephrologist.

This page explains what a test or condition involves. It is not a diagnosis and does not replace advice from a doctor who knows your history. Your results should be interpreted together with your symptoms, other tests and a clinical assessment. Medical disclaimer.