GFR Calculator (MDRD)
Estimate Glomerular Filtration Rate using MDRD and CKD-EPI equations
Patient Information
Valid for adults 18+ years
Normal: 0.6-1.2 mg/dL (53-106 μmol/L)
MDRD Formula
When to use MDRD:
- GFR < 60 (more accurate)
- Diabetic nephropathy
- Standard CKD monitoring
CKD-EPI Formula
When to use CKD-EPI:
- GFR > 60 (more accurate)
- General screening
- 2021+ recommended by NKF/ASN
CKD Stage Classification (KDIGO 2012)
| Stage | GFR (mL/min/1.73m²) | Description | Clinical Action |
|---|---|---|---|
| G1 | ≥ 90 | Normal/High (with kidney damage) | Monitor, control risk factors |
| G2 | 60-89 | Mild decrease (with kidney damage) | Assess progression, reduce CV risk |
| G3a | 45-59 | Mild-Moderate decrease | Nephrology referral if progressing |
| G3b | 30-44 | Moderate-Severe decrease | Nephrology referral recommended |
| G4 | 15-29 | Severe decrease | Prepare for kidney replacement therapy |
| G5 | < 15 | Kidney Failure (ESKD) | Dialysis or transplant required |
Understanding GFR Calculation
What is GFR?
Glomerular Filtration Rate (GFR) is the best overall indicator of kidney function. It measures how much blood passes through the glomeruli (tiny filters in the kidneys) each minute. Normal GFR is 90-120 mL/min/1.73m². A GFR below 60 for 3+ months indicates chronic kidney disease (CKD).
MDRD vs CKD-EPI Formula
| Factor | MDRD (1999) | CKD-EPI (2009) |
|---|---|---|
| Accuracy GFR > 60 | Underestimates (less accurate) | ✅ More accurate |
| Accuracy GFR < 60 | ✅ More accurate | Good, slightly less precise |
| Current Recommendation | Legacy use (diabetic nephropathy) | ✅ Preferred (NKF/ASN 2021) |
| Best Use Case | Established CKD (GFR 15-59) | General screening, all GFR ranges |
Calculation Example
Example Patient:
- 65-year-old white female
- Serum creatinine: 1.4 mg/dL
MDRD Calculation:
GFR = 175 × (1.4)^-1.154 × (65)^-0.203 × 0.742
GFR = 175 × 0.8605 × 0.3926 × 0.742 = 44.1 mL/min/1.73m²
Stage G3b: Moderate-Severe CKD → Nephrology referral recommended
When to Repeat GFR Testing
GFR ≥ 60 (G1-G2)
Every 12 months (if no albuminuria)
GFR 45-59 (G3a)
Every 6-12 months
GFR 30-44 (G3b)
Every 3-6 months
GFR < 30 (G4-G5)
Every 1-3 months (nephrology care)
Limitations and Cautions
⚠️ GFR may be inaccurate in:
- Acute kidney injury (creatinine rising/falling rapidly)
- Extremes of body size (very obese, very thin, amputees)
- Extremes of muscle mass (bodybuilders: overestimate CKD; malnutrition: underestimate)
- Pregnancy (GFR increases 40-65% naturally)
- Vegetarian/vegan diet (lower creatinine → overestimate GFR)
- Recent meat consumption (cooked meat → temporarily ↑ creatinine)
In these cases, consider measured GFR (24-hour urine creatinine clearance) or cystatin C-based eGFR.
2021 Race-Free CKD-EPI Update
In September 2021, the National Kidney Foundation (NKF) and American Society of Nephrology (ASN) recommended removing the race coefficient from GFR equations, citing concerns about perpetuating health disparities. The new CKD-EPI 2021 equation uses the same formula for all races (multiplier = 1.0 for everyone).
Impact: Black patients' eGFR drops by ~4 mL/min on average with the race-free equation, which may:
- ✅ Earlier CKD diagnosis → earlier intervention & nephrology referral
- ✅ Faster transplant waitlist eligibility (GFR < 20 required)
- ⚠️ More patients classified as CKD → increased healthcare utilization
This calculator uses the original 2009 CKD-EPI with race coefficient, as most labs still report this version as of 2024. Check with your lab for which equation they use.
Frequently Asked Questions
What is a normal GFR for my age?
Normal GFR is 90-120 mL/min/1.73m² for young adults. GFR naturally declines ~1 mL/min/year after age 40. Expected GFR by age: Age 20-29: 116 ± 20 | Age 40-49: 99 ± 20 | Age 60-69: 85 ± 20 | Age 70+: 75 ± 20. However, GFR < 60 is never normal, even in elderly, and warrants evaluation if persistent for 3+ months.
How accurate is eGFR compared to measured GFR?
Estimated GFR (eGFR) from creatinine is within ±30% of measured GFR (24-hour urine collection or iothalamate clearance) in 90% of patients. CKD-EPI is accurate within ±10% for GFR 60-89, but less precise for GFR > 90 (may underestimate by 10-15%). For critical decisions (living kidney donation, chemotherapy dosing), measured GFR or cystatin C-based eGFR is recommended.
Why do MDRD and CKD-EPI give different results?
MDRD systematically underestimates GFR > 60 by 6-10 mL/min (may misclassify healthy people as CKD stage 2). CKD-EPI is more accurate at higher GFR ranges. At GFR < 60, results are usually within 2-5 mL/min of each other. Example: 55-year-old male, creatinine 1.0 mg/dL → MDRD: 86 (stage G2), CKD-EPI: 95 (stage G1). For screening, use CKD-EPI; for established CKD monitoring, either is acceptable.
What causes low GFR besides kidney disease?
Reversible causes (pseudoCKD): Dehydration (can drop GFR 20-30%), ACE inhibitor/ARB medications (↓ GFR 10-30% acutely but protective long-term), NSAIDs (ibuprofen, naproxen), muscle breakdown (rhabdomyolysis → ↑ creatinine), recent high-protein meal. Chronic causes: Diabetes (40% of CKD), hypertension (30% of CKD), glomerulonephritis, polycystic kidney disease, prolonged obstruction (BPH, kidney stones). Always repeat GFR after 3 months to confirm CKD diagnosis.
When should I see a nephrologist?
Immediate referral (G4-G5): GFR < 30, proteinuria > 1 g/day, or rapid GFR decline (> 5 mL/min/year). Consider referral (G3b): GFR 30-44, progressive decline, diabetes + albuminuria, or refractory hypertension. Monitor in primary care (G1-G3a): GFR ≥ 45 with stable labs and controlled blood pressure. CKD progression is reversible in early stages (G1-G2) with SGLT2 inhibitors, ACE/ARB, and BP control (< 130/80).
How can I improve my GFR naturally?
Blood pressure control: Keep BP < 130/80 (target < 120/75 if proteinuria) — this is the #1 intervention. SGLT2 inhibitors: Medications like empagliflozin slow CKD progression 30-40% (even non-diabetics). Dietary changes: Limit sodium < 2,300 mg/day, reduce protein to 0.8 g/kg/day if GFR < 60, avoid NSAIDs. Lifestyle: Stop smoking (doubles CKD progression), achieve healthy BMI (obesity worsens GFR), exercise 150 min/week. Monitor: Control diabetes (A1C < 7%), avoid nephrotoxins (contrast dye, aminoglycosides).
References
- Levey AS, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009;150(9):604-612. (CKD-EPI equation)
- Levey AS, et al. A more accurate method to estimate glomerular filtration rate from serum creatinine. Ann Intern Med. 1999;130(6):461-470. (MDRD equation)
- KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int Suppl. 2013;3(1):1-150.
- Delgado C, et al. A Unifying Approach for GFR Estimation: Recommendations of the NKF-ASN Task Force on Reassessing the Inclusion of Race in Diagnosing Kidney Disease. Am J Kidney Dis. 2021;79(2):268-288.
- National Kidney Foundation. GFR Calculator. Available at: kidney.org/professionals/kdoqi/gfr_calculator
Medical Disclaimer: This calculator provides estimated GFR for educational purposes only and should not replace professional medical advice. GFR estimates may be inaccurate in certain populations (see limitations above). Always consult with a healthcare provider for diagnosis and treatment of kidney disease. A single low GFR does not confirm CKD — repeat testing after 3 months is required per KDIGO guidelines.