Kidney Function Tests
Blood, urine, and at-home options, and what each one tells you.
An overview of the tests used to assess kidney function.
Medically reviewed by Dr. Rishi Kumar Kafle, MBBS, MD, FASN. Last reviewed October 2026.
Kidney function is assessed through several tests rather than one, because each marker has blind spots. The core blood markers are creatinine and the eGFR calculated from it, supported by urine tests and ratios like the BUN-to-creatinine ratio.
The Two Things Kidney Tests Measure
Most kidney testing answers one of two questions: how well are the kidneys filtering, and is anything leaking through that shouldn't be. Filtration is captured by creatinine, the eGFR calculated from it, BUN, and cystatin C. Leakage, usually protein or albumin in the urine, is captured by urine tests such as the albumin-to-creatinine ratio. A complete picture often needs both.
| Test | What it measures | Sample |
|---|---|---|
| Creatinine | Muscle waste the kidneys filter; the core marker | Blood |
| eGFR | Estimated filtration rate, calculated from creatinine | Blood (calculated) |
| BUN | Urea nitrogen; also reflects hydration and protein | Blood |
| Cystatin C | Muscle-independent filtration marker | Blood |
| uACR | Albumin leaking into urine: early kidney strain | Urine |
Explore the Tests
- Blood tests for kidney function: creatinine, eGFR, BUN, and cystatin C
- How to check kidney function at home: at-home creatinine and uACR kits, and their limits
- Renal function panel explained: the BMP components and what they show
Why No Single Test Is Enough
No single test is enough because each kidney marker has blind spots. Creatinine is affected by muscle mass, BUN by hydration and diet, and a normal blood test can miss early protein leakage that only a urine test reveals. Kidney health is therefore judged from a combination of results read together, alongside blood pressure, history, and repeat measurements over time. A single value shows one moment, while a trend across repeat results is far more informative.
Blood Tests vs Urine Tests
Blood tests (creatinine, eGFR, BUN, and cystatin C) measure how well the kidneys filter waste from the bloodstream. Urine tests, such as the albumin-to-creatinine ratio, look for substances leaking through the filters that should normally stay in the blood. Healthy kidneys keep protein in; even small amounts of albumin in the urine can be an early warning before filtration numbers change. Because the two test types answer different questions, a thorough assessment usually includes both rather than relying on a blood draw alone.
How Often Kidney Function Is Checked
For most people, kidney function is checked during routine blood work or when starting medications that depend on the kidneys. People with diabetes, high blood pressure, or known chronic kidney disease are monitored more regularly, since these conditions are the main drivers of decline. Repeat testing matters more than any one reading: a value that is stable over months is reassuring, while a steady rise in creatinine or fall in eGFR is what prompts closer attention. Some situations call for a dedicated check, such as testing kidney function before CT contrast, surgery, or nephrotoxic drugs. Always have results interpreted by a clinician in the context of your overall health.
Filtration Rate vs Clearance
Filtration rate and clearance are two related estimates of kidney function. eGFR estimates the glomerular filtration rate and is the standard way to stage kidney health. Creatinine clearance is a closely related estimate used mainly for drug dosing. Both start from a creatinine value and translate it into a measure of how much blood the kidneys clear per minute, but they serve slightly different purposes. Knowing which one a result refers to helps avoid confusion when comparing reports.
Putting It All Together
To put kidney tests together, a clinician reads several of these results at once. The creatinine and eGFR show filtration, the BUN and ratio add hydration context, a urine test checks for protein leakage, and the trend across repeat results shows change over time. No single value diagnoses or rules out kidney disease, and a normal-looking number can sit alongside an early problem that only the urine test reveals. Reading results in layers is what makes kidney testing reliable, so results are best interpreted by a professional who can weigh them against your history. If a result sits outside the usual range, our kidney health guides explain the everyday steps, such as hydration, diet, and blood-pressure control, worth discussing with your care team.
Together these results show how well the kidneys are filtering and whether anything is leaking through. Start with blood tests for kidney function, or estimate clearance from a creatinine result with the creatinine clearance calculator.