Lab Interpretation Guide
Kidney Function Tests — Complete Guide
eGFR · Creatinine · Urea · Uric Acid · Cystatin C · CKD Staging
Quick Answer
eGFR is the primary marker for kidney function staging. eGFR < 60 ml/min/1.73m² persisting for more than 3 months defines chronic kidney disease (CKD). Creatinine and urea support assessment but are insensitive for early impairment — eGFR + urine ACR together give the full picture.
Key Renal Markers
eGFR (CKD-EPI)
Normal: > 90 ml/min/1.73m²
Best overall marker for kidney function staging. Calculated from creatinine, age, and sex. Preferred over creatinine alone for CKD diagnosis and monitoring.
Creatinine
Normal: 62–115 µmol/L (M) · 44–97 (F)
Rises only after ~50% nephron loss — insensitive for early CKD. Affected by muscle mass, diet, and medications. Always calculate eGFR from creatinine.
Urea (BUN)
Normal: 2.5–8.1 mmol/L
Less specific than creatinine. Also elevated in dehydration, high-protein diet, and GI bleeding. Urea:Creatinine ratio > 100 suggests prerenal azotemia.
Uric Acid
Normal: 210–420 µmol/L (M) · 150–350 (F)
Elevated in gout, renal impairment, and metabolic syndrome. Also an independent cardiovascular risk marker. > 480 µmol/L increases gout risk significantly.
Cystatin C
Normal: < 1.15 mg/L
More sensitive than creatinine for early CKD. Not affected by muscle mass. Preferred in elderly patients, low muscle mass, or when creatinine seems discordant.
Urine ACR
Normal: < 3 mg/mmol
Albumin-to-creatinine ratio in urine. As important as eGFR for CKD staging. A3 (> 30 mg/mmol) significantly increases progression risk and CV mortality.
CKD Staging by eGFR
Stage
eGFR
Clinical action
G1 — Normal
≥ 90
CKD only if other kidney damage markers present (proteinuria, hematuria, structural abnormality)
G2 — Mildly decreased
60–89
Monitor if persistent with albuminuria or structural abnormality. Lifestyle counseling.
G3a — Mild–moderate
45–59
Increased CV risk. Consider nephrology referral. Review medications. Monitor BP and proteinuria.
G3b — Moderate–severe
30–44
Nephrology evaluation recommended. Medication dose adjustments required. Anemia workup.
G4 — Severely decreased
15–29
Prepare for renal replacement therapy. Urgent nephrology. Restrict nephrotoxic drugs.
G5 — Kidney failure
< 15
Dialysis or transplant evaluation urgently required. All nephrotoxic drugs stopped.
G1 — Normal≥ 90
CKD only if other kidney damage markers present (proteinuria, hematuria, structural abnormality)
G2 — Mildly decreased60–89
Monitor if persistent with albuminuria or structural abnormality. Lifestyle counseling.
G3a — Mild–moderate45–59
Increased CV risk. Consider nephrology referral. Review medications. Monitor BP and proteinuria.
G3b — Moderate–severe30–44
Nephrology evaluation recommended. Medication dose adjustments required. Anemia workup.
G4 — Severely decreased15–29
Prepare for renal replacement therapy. Urgent nephrology. Restrict nephrotoxic drugs.
G5 — Kidney failure< 15
Dialysis or transplant evaluation urgently required. All nephrotoxic drugs stopped.
Key Clinical Points
- •CKD diagnosis requires abnormality persisting > 3 months — single elevated creatinine may be acute kidney injury
- •Urea:Creatinine ratio > 100 suggests prerenal azotemia (dehydration, reduced perfusion) — rehydrate before repeat
- •Urine ACR is as important as eGFR — high proteinuria with normal eGFR still warrants nephrology referral
- •Medications requiring dose adjustment or cessation in CKD: metformin (stop if eGFR < 30), NSAIDs, aminoglycosides, contrast agents
- •Acute kidney injury (AKI): creatinine rise ≥ 26 µmol/L within 48h, or ≥ 1.5× baseline within 7 days
- •Cystatin C is preferred over creatinine in elderly patients — muscle wasting falsely normalizes creatinine-based eGFR
- •Hyperkalemia (K > 5.5) in CKD is a medical emergency — requires urgent management
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