The short answer
When you look at a laboratory report, your estimated glomerular filtration rate (eGFR)—an estimate of how well your kidneys filter waste—is calculated using mathematical formulas rather than measured directly. The two most common equations are MDRD (Modification of Diet in Renal Disease) and CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration).
While both use your blood creatinine level, age, and sex, CKD-EPI is generally considered more accurate, especially when kidney function is normal or only mildly reduced. Seeing different eGFR numbers on different lab reports often simply reflects a shift to the newer, more precise equation.
What this means
Your eGFR is calculated by plugging your routine blood test results into a standard formula. Creatinine is a natural waste product produced by your muscles and filtered out by your kidneys. Because muscle mass and filtration efficiency vary based on age and sex, laboratories factor those details into the equation.
For many years, laboratories relied primarily on the MDRD equation. Over time, nephrologists and researchers found that MDRD tended to underestimate higher levels of kidney function, often reporting lower numbers than what was actually happening in patients with mild or no kidney disease.
Why the differences matter
The transition from MDRD to CKD-EPI changed how mild kidney impairment is identified:
- Accuracy at higher ranges: CKD-EPI is more precise when your eGFR is above $60\text{ mL/min/1.73 m}^2$. MDRD often reported false positives, making kidney function look worse than it was in healthy individuals or people with early-stage changes.
- Clinical classification: A person might have an eGFR of $58\text{ mL/min/1.73 m}^2$ on an older MDRD report, but cross into the normal or borderline range (e.g., $62\text{ mL/min/1.73 m}^2$) when recalculated with CKD-EPI.
- Consistency: Most modern laboratories and health systems worldwide have transitioned entirely to CKD-EPI to provide more reliable tracking over time.
What a single eGFR number can and cannot tell you
An eGFR score is a valuable screening tool, but it has distinct limitations:
- It is an estimate, not a direct measurement: The math assumes a standard body surface area and average muscle mass. If you have unusually high or low muscle mass (such as bodybuilders, amputees, or individuals with severe muscle wasting), creatinine-based estimates can be skewed.
- It fluctuates: Hydration status, heavy meat consumption, intense exercise right before a blood draw, and certain medications can temporarily raise blood creatinine and lower your calculated eGFR.
- One number lacks context: A single low reading does not automatically confirm chronic kidney disease. Clinicians look for persistent trends over three months or more.
What you can do next
- Check which equation was used: Look at the fine print on your lab report to see whether “CKD-EPI” or “MDRD” is listed. Most modern labs display CKD-EPI by default.
- Track the trend, not just the snapshot: Compare current results against past laboratory reports rather than focusing on a small numerical shift between two tests.
- Discuss results with your clinician: If you notice a change in your eGFR or if your lab report shows a shift between formulas, review it at your next check-up alongside other vital markers like blood pressure and urine protein tests (albuminuria).
Bottom line
The differences between the MDRD and CKD-EPI formulas come down to mathematical precision. CKD-EPI provides a more reliable picture of your kidney filtration, particularly in the normal and mildly reduced ranges. If your numbers have shifted slightly between lab visits, check to see if a formula update explains the change.