What a kidney function test includes
Your kidneys contain tiny filters called glomeruli that remove waste and extra water from the blood and send it out in the urine. Because early kidney disease usually causes no symptoms, NIDDK points out that testing is the only way to know how well your kidneys are working. A kidney function test panel usually includes:
- Serum creatinine, a waste product of normal muscle activity that the kidneys remove from the blood;
- Blood urea nitrogen (BUN) or blood urea, the waste that forms when protein is broken down;
- eGFR (estimated glomerular filtration rate), a number calculated from your creatinine and personal details such as age and sex;
- Uric acid, which some labs add to the panel because high levels can cause gout or kidney problems.
Doctors often pair the blood test with a urine test for albumin, as NIDDK describes below. The exact list differs from lab to lab, so check the test names on your report. At our centre, the diagnostics catalogue lists the Renal Function Test (RFT) as well as separate creatinine, blood urea nitrogen (BUN) and uric acid tests.
Kidney function test normal range table
These are the common adult ranges quoted by MedlinePlus. Your lab may use slightly different numbers or units.
| Test | Common normal range (adults) | Source |
|---|---|---|
| Serum creatinine, men | 0.7 to 1.3 mg/dL (62 to 115 µmol/L) | MedlinePlus |
| Serum creatinine, women | 0.5 to 0.95 mg/dL (44 to 84 µmol/L) | MedlinePlus |
| Blood urea nitrogen (BUN) | Generally 6 to 20 mg/dL | MedlinePlus |
| eGFR | 60 or more is in the normal range (NIDDK); 90 to 100 means you probably do not have kidney disease (MedlinePlus) | NIDDK, MedlinePlus |
| Uric acid, women | 3.0 to 7.1 mg/dL | MedlinePlus |
| Uric acid, men | 4.0 to 8.6 mg/dL | MedlinePlus |
| Urine albumin-to-creatinine ratio (UACR) | 30 mg/g or less is normal | NIDDK |
Women usually have a lower creatinine level than men because they often have less muscle mass, and creatinine also varies with body size. That is why a single creatinine number is rarely read on its own.
Creatinine: the main kidney marker
Creatinine comes from creatine, a chemical your muscles use for energy. It is removed from the body almost entirely by the kidneys, so if kidney function falls, creatinine builds up in the blood. According to MedlinePlus, a higher than normal level may be due to a blocked urinary tract, kidney damage, infection or reduced blood flow to the kidneys, loss of body fluid (dehydration), breakdown of muscle, or pregnancy complications such as pre-eclampsia. A lower than normal level may be linked to reduced muscle mass or malnutrition.
Some medicines can affect the result, including cimetidine, famotidine and certain antibiotics such as trimethoprim. Your doctor may ask you to pause a medicine before the test, but never stop one on your own.
Urea and BUN: why your report may show either
Urea nitrogen forms when protein breaks down. Some reports show BUN (blood urea nitrogen) and others show blood urea. The two are related but are not reported as the same number, so compare each one only with the range printed beside it.
MedlinePlus lists several reasons for a higher than normal BUN, including dehydration, a very high-protein diet, bleeding in the stomach or intestines, heart failure, a heart attack, kidney disease and a blocked urinary tract. A low level may be seen with liver failure, a low-protein diet, malnutrition or drinking a lot of fluid. In people with liver disease, BUN can be low even when the kidneys are normal, which is one reason doctors look at the whole panel together, often alongside a liver function test.
eGFR: what the number tells you
Directly measuring kidney filtration is complicated and rarely done, so laboratories estimate it. MedlinePlus explains that eGFR is calculated with a formula that combines your creatinine level with details such as your age and sex. Some labs use a protein called cystatin C instead of, or together with, creatinine, because it is less affected by muscle size and diet.
- 60 or more: in the normal range, according to NIDDK. MedlinePlus adds that a result between 60 and 90 may point to early-stage kidney disease, so ask your doctor what it means for you.
- Below 60: may mean kidney disease.
- 15 or less: is called kidney failure. NIDDK says most people below this level need dialysis or a kidney transplant.
NIDDK also notes that you cannot raise your GFR, but you can try to keep it from going lower. An eGFR is not a perfect test, and one abnormal result can occur without kidney damage, so your doctor may order more tests to confirm it.
The urine albumin test (UACR)
A healthy kidney does not let albumin, a blood protein, pass into the urine. NIDDK explains that doctors check for kidney damage with a urine albumin-to-creatinine ratio (UACR). A result of 30 mg/g or less is normal, and more than 30 mg/g may be a sign of kidney disease. If albumin is found, the test is usually repeated one or two more times to confirm it. For a full picture, doctors use both the eGFR blood test and the urine albumin test.
Is fasting needed for a KFT? How to prepare
Fasting is not always needed for a kidney function test, but some labs ask for it. MedlinePlus says you may need to fast, or avoid certain foods such as meat, for several hours before an eGFR test, and that you should not eat or drink for 4 hours before a uric acid test unless told otherwise. A simple way to prepare:
- Ask the lab about fasting. Ask the lab or your doctor whether to fast and for how long. If other tests such as fasting sugar are ordered on the same sample, you may need a longer fast; see our fasting blood test guide.
- Avoid a heavy meat meal. If you are asked to, avoid meat for several hours before the test, because it can affect the eGFR result.
- List your medicines. Write down every medicine and supplement you take and show the list to the doctor or lab. Do not stop any medicine unless your doctor tells you to.
- Drink water normally. Unless you are told otherwise, drink plain water as usual, because dehydration can raise creatinine and BUN.
- Keep your old reports. Bring previous kidney reports so the doctor can compare the trend, not just one value.
Who should get a kidney function test?
NIDDK advises getting checked for kidney disease if you have diabetes, high blood pressure, heart disease or a family history of kidney failure. If you have diabetes, it recommends a check every year; for the other conditions, ask your doctor how often. MedlinePlus adds being over 60, smoking and obesity as risk factors.
Later stages of kidney disease can cause symptoms such as swelling in the legs, feet, ankles, hands or face, passing urine more or less often than usual, dry or itchy skin, tiredness, muscle cramps, nausea and loss of appetite. If you notice these, see a doctor rather than waiting for a routine check.
People with diabetes, who NIDDK advises to have a kidney check every year, may also want to track their HbA1c and lipid profile.