Blood Tests for Kidney Function
Creatinine, eGFR, BUN, and cystatin C: what each one measures.
The blood tests used to assess kidney function and what they reveal.
Medically reviewed by Dr. Rishi Kumar Kafle, MBBS, MD, FASN. Last reviewed October 2026.
Several blood tests assess kidney function, and each one measures a different part of it. Together they show how effectively the kidneys clear waste from the blood.
The Main Blood Markers
| Test | What it measures |
|---|---|
| Creatinine | Muscle waste filtered by the kidneys; the core marker |
| eGFR | Estimated filtration rate, calculated from creatinine |
| BUN | Urea nitrogen; also affected by hydration and protein intake |
| Cystatin C | A muscle-independent marker, useful when creatinine is unreliable |
What Each Test Measures
Creatinine is a waste product made steadily by muscle and filtered out by the kidneys, so a higher blood level usually means slower filtration. eGFR takes that creatinine value, along with age and sex, and converts it into an estimated filtration rate that is easier to interpret. BUN measures urea nitrogen, another waste product, but it is also swayed by hydration, protein intake, and bleeding in the gut. It is therefore read as context rather than a stand-alone kidney number. Cystatin C is filtered like creatinine but is largely independent of muscle mass, so it is useful when creatinine may mislead.
How the Sample Is Taken
All four are measured from a routine blood draw, usually from a vein in the arm, and often as part of a wider renal function panel or basic metabolic panel. No special preparation is normally required, though your clinician may ask about recent heavy exercise, large protein meals, or muscle-building supplements, since these can nudge the numbers. Some people screen first with an at-home kidney test kit, but a laboratory draw remains the reference that confirms or rules out a home result.
How They Fit Together
Creatinine is measured directly, then eGFR is calculated from it to express function as a filtration rate. BUN adds context about hydration, and cystatin C helps when muscle mass distorts creatinine. Clinicians often look at the BUN-to-creatinine ratio to separate dehydration from intrinsic kidney problems.
Limitations to Keep in Mind
Blood tests are a snapshot. Creatinine depends on muscle mass, so it can read low in people with little muscle and high in very muscular people, even when filtration is normal. A single result can also be skewed by dehydration or a recent large protein meal. For these reasons, results are best interpreted as a trend over time and alongside a urine test for protein, which blood tests alone do not capture.
When Cystatin C Is Useful
Most kidney testing starts with creatinine because it is inexpensive and widely available, but cystatin C is useful when creatinine may mislead. Since creatinine depends on muscle mass, it can read deceptively low in people with little muscle, such as older or frail patients, and high in very muscular people. Cystatin C is largely independent of muscle, so it offers a second view of filtration, sometimes used to confirm borderline results or to estimate eGFR more accurately in these groups. A clinician decides when the extra test adds value.
What Affects the Results
A few everyday factors can shift these numbers without meaning kidney disease. Heavy exercise, a large cooked-meat meal, dehydration, and certain muscle-building supplements can all nudge creatinine up temporarily. BUN swings with hydration and protein intake more than creatinine does. Because of this, an unexpected result is often rechecked, and clinicians interpret values in the context of your recent activity, diet, and previous readings rather than reacting to one number.
Reading Results as a Trend
A single blood test is a snapshot, and the most useful information often comes from comparing it with earlier results. A creatinine that has been stable for years is reassuring even if it sits slightly outside a population reference range. A value climbing steadily over months is a clearer warning, even if each individual reading looks acceptable. For this reason, clinicians weigh the direction of change as well as whether a number falls inside a range, and they may repeat a surprising result before drawing conclusions. A one-off test is also ordered for specific clinical reasons, such as checking kidney function before contrast scans, surgery, or nephrotoxic drugs.
Blood Tests vs Urine Tests
Blood markers describe how well the kidneys filter, but they miss a second, equally important question: whether the filter is leaking. A damaged glomerulus can start letting protein slip into the urine well before filtration measurably drops, so a normal creatinine and eGFR do not rule out early kidney damage on their own. That gap is filled by a urine test, most often the urine albumin-to-creatinine ratio, which detects small amounts of leaked albumin. A complete kidney assessment therefore usually pairs the blood markers above with a urine sample. The blood markers measure filtration and the urine test measures leakage, and chronic kidney disease is staged using both together.
To turn a creatinine result into a clearance estimate, use the creatinine clearance calculator, or compare a value with the creatinine normal range.