How is Chronic Kidney Disease (CKD) stage computed?

How to Compute CKD Stage

CKD staging is determined by calculating estimated glomerular filtration rate (eGFR) using validated equations and assessing albuminuria, then classifying patients into stages G1-G5 based on eGFR thresholds combined with albuminuria categories A1-A3. 1

Step 1: Calculate eGFR

Use the CKD-EPI equation as the preferred method for estimating GFR in adults, as it provides superior accuracy compared to the older MDRD equation, particularly at GFR >60 mL/min/1.73 m². 2 The MDRD equation is acceptable but tends to underestimate GFR at higher levels. 3, 4

Recommended Equations:

For Adults:

• CKD-EPI equation (preferred): More accurate across all GFR ranges, especially >60 mL/min/1.73 m² 2, 3, 4
• Simplified MDRD equation: GFR (mL/min/1.73m²) = 186 × [serum creatinine (mg/dL)]^-1.154 × [age (years)]^-0.203 × [0.742 if female] × [1.212 if black] 2
• Cockcroft-Gault equation: CrCl (mL/min) = [140 - age (years)] × weight (kg) [×0.85 if female] / (72 × serum creatinine (mg/dL)) - primarily used for medication dosing decisions 2

For Children:

• Schwartz formula or Counahan-Barratt equation 2

Critical Caveat:

The Cockcroft-Gault equation estimates creatinine clearance (not GFR) and is most appropriate when determining medication dosages in renal failure, as drug studies traditionally used this formula. 2 For CKD staging itself, use CKD-EPI or MDRD equations. 2

Step 2: Assign GFR Category (G Stage)

Classify based on eGFR value (mL/min/1.73 m²): 2, 1

• G1 (Normal or high): eGFR ≥90
• G2 (Mildly decreased): eGFR 60-89
• G3a (Mildly to moderately decreased): eGFR 45-59
• G3b (Moderately to severely decreased): eGFR 30-44
• G4 (Severely decreased): eGFR 15-29
• G5 (Kidney failure): eGFR <15 or on dialysis

Step 3: Measure Albuminuria

Obtain a spot urine albumin-to-creatinine ratio (UACR) - this is more practical and accurate than 24-hour urine collections. 2

Albuminuria Categories (A Stage): 1

• A1 (Normal to mildly increased): UACR <30 mg/g
• A2 (Moderately increased/microalbuminuria): UACR 30-299 mg/g
• A3 (Severely increased/macroalbuminuria): UACR ≥300 mg/g

Sex-specific thresholds may be used: >17 mg/g in men and >25 mg/g in women for abnormal albuminuria. 2

Step 4: Confirm Chronicity

Both kidney damage markers and reduced GFR must persist for ≥3 months to diagnose CKD. 2 A single abnormal measurement is insufficient - repeat testing is mandatory.

Step 5: Apply Diagnostic Criteria

For Stages G1-G2 (eGFR ≥60):

Evidence of kidney damage is REQUIRED to diagnose CKD. 1 eGFR alone is insufficient. Required markers include: 2, 1

• Albuminuria (UACR ≥30 mg/g)
• Pathological abnormalities on biopsy
• Structural abnormalities on imaging (cysts, scarring)
• History of kidney transplantation

For Stages G3-G5 (eGFR <60):

eGFR <60 mL/min/1.73 m² alone is sufficient to diagnose CKD, even without albuminuria or other markers of kidney damage. 2, 1 This threshold represents loss of half or more of normal adult kidney function and is associated with increased complications. 2

Step 6: Combine into CGA Classification

The complete CKD stage combines both G and A categories (e.g., G3aA2, G4A3). 1 This matrix provides superior risk stratification for:

• CKD progression
• Cardiovascular events
• Mortality

Risk Stratification: 1

• Green (Low risk): G1A1, G2A1
• Yellow (Moderately high risk): G1A2, G2A2, G3aA1
• Orange (High risk): G1A3, G2A3, G3aA2, G3bA1
• Red (Very high risk): G3aA3, G3bA2, G3bA3, G4A1-A3
• Dark red (Highest risk): G5A1-A3

Common Pitfalls to Avoid

Do not rely on serum creatinine alone - it grossly overestimates kidney function and can miss significant CKD. 2 A "normal" creatinine of 1.3 mg/dL can coexist with substantially reduced GFR. 2

Do not use 24-hour creatinine clearance - it is less accurate than MDRD/CKD-EPI equations, prone to collection errors, and burdensome for patients. 2

Age considerations: GFR naturally declines with age (~17% of people >60 years have eGFR <60), but this still represents CKD requiring appropriate management and drug dosing adjustments. 2 The definition of CKD is the same regardless of age. 2

Ethnicity matters: The MDRD and CKD-EPI equations include race-based adjustments (multiply by 1.212 for Black patients). 2 Failure to apply these corrections leads to inaccurate staging.

When to Refer to Nephrology

Immediate nephrology referral is indicated for: 1

• eGFR <30 mL/min/1.73 m² (stages G4-G5)
• Any stage with A3 albuminuria (≥300 mg/g)
• Rapidly declining eGFR or rapidly increasing albuminuria
• Active urinary sediment or nephrotic syndrome
• Absence of diabetic retinopathy in type 1 diabetes with kidney disease