What eGFR Measures
Glomerular filtration rate is the volume of blood the kidneys filter per minute, normalised to a standard body surface area of 1.73 m². Measuring it directly requires injecting a marker substance, so in practice it is estimated from serum creatinine — a muscle breakdown product the kidneys clear at a predictable rate.
The CKD-EPI 2021 equation is the current standard in the United States and increasingly elsewhere. It replaced earlier versions by removing the race coefficient, which had produced systematically higher estimates for Black patients without a physiological basis.
Kidney Disease Stage Chart
| Stage | eGFR | Description | Typical management |
|---|---|---|---|
| G1 | 90 or above | Normal or high | Only CKD if damage markers are present |
| G2 | 60–89 | Mildly decreased | Only CKD if damage markers are present |
| G3a | 45–59 | Mild to moderate | Monitoring, blood pressure and risk factor control |
| G3b | 30–44 | Moderate to severe | Nephrology referral is common |
| G4 | 15–29 | Severely decreased | Preparation for replacement therapy |
| G5 | Under 15 | Kidney failure | Dialysis or transplant usually required |
Stages 1 and 2 require evidence of kidney damage — albuminuria, imaging abnormalities or biopsy findings — because a normal GFR alone is not disease.
What Creatinine Depends On
Creatinine comes from muscle, so the estimate is affected by anything that changes muscle mass or turnover:
- High muscle mass raises creatinine and lowers the estimated GFR, so bodybuilders can appear to have reduced kidney function.
- Low muscle mass — older adults, amputees, people with chronic illness — does the reverse, masking genuine impairment.
- Cooked meat raises creatinine for hours, which is why fasting samples are preferred.
- Creatine supplements raise it modestly without affecting kidney function.
- Dehydration raises it temporarily.
Where creatinine is unreliable, cystatin C provides an independent estimate that does not depend on muscle mass. Combining the two is more accurate than either alone.
GFR Declines With Age
Filtration rate falls by roughly 1 mL/min a year after age 40. An eGFR of 65 at 80 is close to expected; the same value at 30 warrants investigation. Age is already in the equation, but the staging thresholds are not age-adjusted, which is why many older adults are classified as stage G2 or G3a without progressive disease.
Frequently Asked Questions
Does one low result mean kidney disease?
No. CKD requires abnormality persisting for at least three months. A single result can reflect dehydration, a recent meal, medication or laboratory variation, and repeating the test is the standard next step.
Can GFR improve?
Acute reductions from dehydration, medication or illness often recover fully. Chronic structural damage generally does not, though controlling blood pressure and blood glucose slows further decline substantially.
Why is my eGFR different between labs?
Creatinine assays vary, and laboratories may use different equations — CKD-EPI 2021, the older 2009 version, or MDRD. Ask which equation was used when comparing results over time.