Creatinine is a sluggish measure: the GFR can halve before the creatinine leaves the reference range, because the relationship is hyperbolic. In people with a low muscle mass — the elderly, the undernourished, amputees, patients with hepatic cirrhosis — creatinine-based eGFR substantially overestimates renal function.
Staging of chronic kidney disease
| Stage | eGFR | Description |
|---|---|---|
| G1 | ≥ 90 | Normal or high |
| G2 | 60–89 | Mildly reduced |
| G3a | 45–59 | Mildly to moderately reduced |
| G3b | 30–44 | Moderately to severely reduced |
| G4 | 15–29 | Severely reduced |
| G5 | < 15 | Kidney failure |
Albuminuria
Albuminuria is an independent risk marker for both kidney failure and cardiovascular death — a patient with an eGFR of 70 and A3 is at higher risk than one with an eGFR of 50 and A1.
Estimating renal function
| Formula | Used for |
|---|---|
| CKD-EPI 2021 (creatinine) | The first choice for eGFR in adults |
| Cystatin C-based eGFR | With an atypical muscle mass, or to confirm a creatinine-based value |
| The mean of creatinine and cystatin C | The most reliable in routine clinical practice |
| Cockcroft–Gault | Still used in certain dosing recommendations — it gives clearance, not GFR |
| Iohexol clearance | The reference method for cytotoxic drug dosing and donor assessment |
For drug dosing: check which measure the dosing recommendation is based on. The absolute GFR (mL/min) must be used for dosing, not the value normalised to body surface area — the difference is substantial in very small and very large people.
Acute kidney injury (KDIGO)
| Stage | Creatinine | Urine output |
|---|---|---|
| 1 | A rise ≥ 26 µmol/L in 48 h, or 1.5–1.9 × the baseline | < 0.5 mL/kg/h for 6–12 h |
| 2 | 2.0–2.9 × the baseline | < 0.5 mL/kg/h for ≥ 12 h |
| 3 | ≥ 3.0 × the baseline, or a creatinine ≥ 354 µmol/L, or the start of dialysis | < 0.3 mL/kg/h for ≥ 24 h, or anuria ≥ 12 h |
Other kidney-related values
The fractional excretion of sodium (FENa) = (urine Na × plasma creatinine) / (plasma Na × urine creatinine) × 100. Below 1 % suggests a prerenal cause, above 2 % acute tubular necrosis — but the value is unreliable during diuretic therapy, where a fractional excretion of urea below 35 % is a better alternative.
Follow-up
At an eGFR below 60, or in the presence of albuminuria: check the potassium, calcium, phosphate, PTH, haemoglobin and bicarbonate, review the medication list (metformin, NSAIDs, RAAS blockade, gabapentin, direct oral anticoagulants) and refer to nephrology at an eGFR < 30, with rapid deterioration, or with A3 albuminuria.
