Short answer
Kidney function testing often starts with blood tests such as creatinine, estimated glomerular filtration rate (eGFR), and BUN. These can help show how well the kidneys are filtering waste from the blood. But kidney assessment is not only a blood-test question: NIDDK says clinicians use both a blood test for GFR and a urine test for albumin to check for kidney disease.
Common kidney-related tests
| Test | Sample | What it helps show |
|---|---|---|
| Creatinine | Blood and/or urine | A waste product from normal muscle activity. If kidneys are not filtering well, creatinine can build up in blood. |
| eGFR | Calculated from blood markers | An estimate of how much blood the kidneys filter each minute, often calculated from creatinine and personal factors such as age and sex. |
| BUN | Blood | Blood urea nitrogen, a waste product from protein breakdown that the kidneys remove. |
| Urine albumin-to-creatinine ratio (UACR) | Urine | A comparison of urine albumin with urine creatinine. Albumin in urine can be a sign of kidney damage. |
| BMP or CMP | Blood | Panels that commonly include creatinine and BUN, along with other chemistry markers. |
Creatinine and eGFR
Creatinine is a normal waste product from muscle use. MedlinePlus notes that creatinine alone is not the best way to check kidney function because levels vary with muscle mass, food, age, and activity. Creatinine is often used to calculate eGFR, which MedlinePlus describes as a more accurate way to measure kidney health than creatinine alone.
BUN
BUN measures urea nitrogen, a waste product made when the body breaks down protein. MedlinePlus says BUN is often part of a comprehensive metabolic panel and can help diagnose or monitor kidney disease. A high BUN can suggest that the kidneys are not working well, but BUN can also be affected by dehydration, certain medicines, a high-protein diet, burns, and other non-kidney factors.
Why urine albumin may be part of the picture
NIDDK says health care providers use two tests to check for kidney disease: a blood test that checks GFR and a urine test for albumin. A urine albumin-to-creatinine ratio can help estimate how much albumin is passing into urine. NIDDK lists UACR of 30 mg/g or less as normal, while more than 30 mg/g may be a sign of kidney disease and may need repeat testing to confirm.
What cystatin C may add
When creatinine is hard to interpret, cystatin C can add another estimate of kidney filtering. NIDDK says the combined creatinine-cystatin C equation can improve accuracy, especially when a decision depends on a better eGFR estimate or when muscle mass may make creatinine less reliable. That is why some kidney evaluations include cystatin C alongside the usual blood and urine markers rather than replacing them.
Who may be checked
Early kidney disease often has no symptoms. MedlinePlus and NIDDK both highlight higher-risk situations such as diabetes, high blood pressure, heart disease, family history of kidney disease or kidney failure, smoking, obesity, and older age. Testing may also be used to monitor known kidney disease or medicines that can affect the kidneys.
Preparation and result caveats
Follow the instructions from the ordering clinician or lab. MedlinePlus says creatinine testing may require avoiding meat for 24 hours beforehand, and that fasting may be needed if creatinine is part of a basic metabolic panel or CMP. Certain medicines and supplements can affect results, so tell the ordering clinician what you take, but do not stop prescribed medicines unless told to.
Questions to ask
- Was my eGFR calculated from creatinine, cystatin C, or both?
- How does this result compare with my prior kidney test results?
- Should I also have a urine albumin-to-creatinine ratio?
- Could dehydration, recent illness, exercise, meat intake, supplements, or medicines have affected the result?
- Would cystatin C help if creatinine-based eGFR seems uncertain?
- Should the test be repeated before labeling this as kidney disease?
Kidney function can also be part of medication monitoring, including some HIV PrEP care. See the PrEP labs and STI testing follow-up guide.
FAQ
What blood tests check kidney function?
Common kidney-related blood tests include creatinine, estimated glomerular filtration rate or eGFR, and BUN. Creatinine and BUN are often included in a basic metabolic panel or comprehensive metabolic panel.
Is eGFR more useful than creatinine alone?
MedlinePlus says eGFR is usually a more accurate way to measure kidney health than creatinine alone because creatinine levels vary with factors such as muscle mass, diet, age, and activity.
When is cystatin C ordered?
Cystatin C may be ordered when creatinine-based eGFR may be less reliable, when muscle mass makes creatinine harder to interpret, or when a more precise kidney estimate is needed for a decision.