What a kidney function test measures

A kidney function test measures how well your kidneys are filtering waste from your blood. The test looks at two main things: the level of creatinine in your blood, and the level of a protein called blood urea nitrogen (BUN). Both of these build up when your kidneys are not working as they should. The test also calculates your glomerular filtration rate, or GFR, which is a number that tells you what percentage of normal kidney function you have left.

Your doctor orders this test from a straightforward blood draw. The lab measures creatinine and BUN, then uses your age, sex, body size, and race to calculate your GFR. That GFR number is what your doctor uses to stage kidney disease if there is one. A GFR of 90 or higher is considered normal. Below 60 means your kidneys are not filtering at the rate they should, and below 15 usually means you may need dialysis or a transplant.

The test does not tell you what is causing a problem — only that one exists. If your results are abnormal, your doctor will order other tests to find the cause, such as an ultrasound of your kidneys or a urine test to check for protein.

Key Takeaways

  • A kidney function test measures creatinine and BUN in your blood and calculates your GFR, which shows what percentage of normal kidney function remains.
  • The test requires only a blood draw and is often part of a routine physical or ordered when you have symptoms like swelling, fatigue, or high blood pressure.
  • Results below a GFR of 60 suggest your kidneys are not filtering waste at a normal rate, but the test does not identify the cause.
  • Your doctor will order follow-up tests if results are abnormal, such as a urine test or kidney ultrasound, to understand what is happening.

Why doctors order kidney function tests

Your doctor may order a kidney function test as part of a routine physical exam, especially if you are over 60 or have a condition that stresses the kidneys. Diabetes and high blood pressure are the two most common causes of kidney disease, so anyone with either condition gets this test regularly. Your doctor may also order it if you have symptoms like swelling in your legs or feet, fatigue that does not go away, or changes in how often you urinate.

The test is also ordered before you start certain medications — some blood pressure drugs, diabetes medications, and pain relievers can affect kidney function, so your doctor needs a baseline measurement. If you are already taking a medication that can harm the kidneys, your doctor will order the test every few months or once a year to make sure the medication is not causing damage.

What the numbers mean

Your GFR is the most important number on a kidney function test report. It is measured in milliliters per minute per 1.73 square meters of body surface area, written as mL/min/1.73m². A GFR of 90 or higher is normal. A GFR between 60 and 89 means your kidneys are working well enough that most people have no symptoms, but your doctor will watch for changes. A GFR between 45 and 59 means mild to moderate kidney disease. A GFR between 30 and 44 means moderate to severe kidney disease. A GFR below 30 means severe kidney disease, and below 15 means kidney failure.

Creatinine is measured in milligrams per deciliter (mg/dL). Normal creatinine is usually between 0.7 and 1.3 mg/dL for men and 0.6 and 1.1 mg/dL for women, though this varies by lab. BUN is measured in mg/dL as well, and normal is usually between 7 and 20 mg/dL. Both creatinine and BUN can be high for reasons other than kidney disease — dehydration, muscle breakdown, or a high-protein diet can raise them — so your doctor looks at the GFR calculation, which accounts for these variables.

How to prepare for the test

A kidney function test requires only a blood draw, so there is little to prepare. You do not need to fast before the test, though some doctors ask you to avoid strenuous exercise for 24 hours beforehand because intense activity can temporarily raise creatinine. Tell your doctor about any medications or supplements you take, because some can affect the results. If you are dehydrated, creatinine can be falsely high, so drink water normally in the days before the test.

The blood draw itself takes a few minutes. A phlebotomist will tie a band around your upper arm, clean the inside of your elbow with an alcohol wipe, and insert a needle into a vein. You may feel a pinch. The blood goes into a tube and is sent to a lab for analysis. Results usually come back within a few days, and your doctor will contact you to discuss them.

What happens if results are abnormal

If your GFR is below 60, your doctor will order follow-up tests to understand why. A urinalysis checks for protein or blood in your urine, which can point to kidney disease. An ultrasound or CT scan of your kidneys shows their size and shape and can reveal scarring, cysts, or blockages. Your doctor may also check your blood pressure, because high blood pressure both causes and results from kidney disease.

If kidney disease is confirmed, your doctor will discuss what is causing it and what you can do to slow its progression. For most people, controlling blood pressure and blood sugar (if you have diabetes) is the first step. Your doctor may prescribe medications that protect the kidneys, such as ACE inhibitors or ARBs. You may be referred to a nephrologist, a doctor who specializes in kidney disease, especially if your GFR is below 30.

Abnormal results do not mean your kidneys are failing. Many people live for years with mild to moderate kidney disease and never need dialysis. Regular monitoring and managing the underlying cause — usually diabetes or high blood pressure — can slow or stop the disease from getting worse.

Kidney function tests in older adults

Kidney function naturally declines with age, so your doctor may interpret results differently for someone over 65 than for a younger person. An older adult with a GFR of 45 may not need the same level of intervention as a younger person with the same number, because the decline may be normal aging rather than disease. However, a sudden drop in GFR — even if the number itself is still in a range considered acceptable for age — is always worth investigating.

Older adults are also more likely to take medications that can affect kidney function, such as NSAIDs for arthritis or certain blood pressure drugs. If you are over 60 and take any regular medications, your doctor should check your kidney function at least once a year. If you have diabetes or high blood pressure, the test should happen more often.

Frequently Asked Questions

Can a kidney function test show kidney stones?

No. A kidney function test measures how well your kidneys filter, not their structure. If your doctor suspects kidney stones, they will order an ultrasound or CT scan instead. However, if a stone is blocking urine flow and damaging your kidneys, the kidney function test will show abnormal results, which prompts your doctor to look for the cause.

What should I do if my GFR is declining but still above 60?

A slow decline over time is more concerning than a single low result. Tell your doctor about any changes in your symptoms — new swelling, increased fatigue, or changes in urination. Make sure your blood pressure and blood sugar (if you have diabetes) are well controlled, because these are the main things you can change. Your doctor will likely order the test again in a few months to see if the decline continues.

Does a kidney function test show whether I have diabetes?

No. A kidney function test shows only how well your kidneys are filtering. However, if you have diabetes and your kidneys are damaged from high blood sugar, the test will show abnormal results. Your doctor uses a separate blood test (fasting glucose or hemoglobin A1C) to diagnose or monitor diabetes.

Why did my creatinine go up between tests?

A single rise in creatinine can happen for many reasons: dehydration, intense exercise, a high-protein diet, or a new medication. Your doctor will ask about these things before concluding your kidney function has actually declined. If creatinine stays high on a second test, or if your GFR drops, that is when your doctor will investigate further.