Your glomerular filtration rate (GFR) estimates how much blood your kidneys filter each minute, and in practice you get it from a blood test: a lab measures creatinine, ideally cystatin C as well, and an equation turns that into an estimated GFR (eGFR). This is a clinic or laboratory measurement, not a home one. You may already have the number: most routine chemistry panels report eGFR next to creatinine, in mL/min/1.73 m².
What you need
- A venous blood draw at a clinic, hospital or collection center.
- Serum creatinine — on almost every basic metabolic or renal panel.
- Cystatin C, if your lab offers it. Usually charged separately, but it makes the estimate more reliable.
- Your age and sex on the day of the test.
- Height and weight only if a clinician needs the result converted off the standard body-surface-area scale for drug dosing.
- Optionally a spot urine sample for the albumin-to-creatinine ratio, the other half of a kidney assessment.
Fasting is not required. In the 24 hours before the draw, avoid a large meat meal, creatine supplements and an unusually hard workout — all three can push creatinine up and drag the estimate down for reasons unrelated to your kidneys.
How the measurement works
True GFR can only be measured directly by tracking how fast the body clears a filtration marker such as inulin or iohexol. That means a timed infusion and repeat sampling, done in specialist settings. Everything else is an estimate.
Creatinine is the classic marker: a waste product of muscle turnover, filtered out at a fairly constant rate. The higher your creatinine, the lower the estimated filtration rate. Age also enters the equation and pushes the estimate down as it rises; sex adjusts for typical differences in muscle mass. The 2021 CKD-EPI equations, now standard in the United States, dropped the older race coefficient. Where Cystatin C is available it is worth adding: it is made by all nucleated cells and barely depends on muscle, and the combined equation is the most accurate in routine use.
The arithmetic is not worth doing by hand. Enter your values into the Glomerular filtration rate (eGFR) calculator, which runs the 2021 creatinine, 2021 creatinine–cystatin C and 2012 cystatin C equations and shows where you land.
How to read the result
The result maps onto the KDIGO filtration categories, G1 to G5 (some tools label them C1 to C5):
| eGFR (mL/min/1.73 m²) | Category | Description |
|---|---|---|
| 90 and above | G1 | Normal or high filtration |
| 60–89 | G2 | Mildly decreased |
| 45–59 | G3a | Mildly to moderately decreased |
| 30–44 | G3b | Moderately to severely decreased |
| 15–29 | G4 | Severely decreased |
| Below 15 | G5 | Kidney failure |
A category is not a diagnosis. Chronic kidney disease requires the abnormality to persist for more than three months, so a single low reading calls for a repeat test rather than a conclusion. G1 and G2 alone are normal filtration; they count as disease only when something else — albumin in the urine, an imaging finding — shows damage alongside them. Trend matters more than any single value: a stable 68 is a different story from a 68 that was 88 two years ago.
What it does not tell you
It does not tell you why. eGFR quantifies filtration, not its cause, and cannot distinguish diabetic kidney disease from an obstruction, a drug effect or simple dehydration. Other tests do that work; a raised BUN/Creatinine ratio, for instance, points upstream of the kidney itself.
It is an estimate, and estimates fail in predictable places. Creatinine tracks muscle, so a heavily muscled person can look worse than they are and a frail or elderly one better. Amputation, a vegetarian diet and creatine supplementation all shift it. Some medicines, trimethoprim and cimetidine among them, raise creatinine by blocking its secretion into urine without touching real filtration. Cystatin C avoids the muscle problem but has its own: thyroid disease, corticosteroids, marked obesity and active inflammation all move it.
The equations assume a steady state, so they are unreliable during acute illness or when kidney function is changing by the day. They were derived in adults and do not apply to children or pregnancy. Normal filtration does not mean healthy kidneys: albuminuria often appears while eGFR is still comfortably in range, so the urine test belongs beside the blood test, not after it.
Related reading
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.