How do you classify Chronic Kidney Disease (CKD) stages?

How to Classify Chronic Kidney Disease (CKD) Stages

CKD is classified using the KDIGO CGA system, which combines three components: Cause of kidney disease, GFR category (G1-G5), and Albuminuria category (A1-A3), creating a comprehensive risk stratification matrix. 1, 2

GFR Categories (G Stages)

The five GFR stages are defined by estimated glomerular filtration rate (eGFR) measured in mL/min/1.73 m²: 1, 2

• G1 (Normal or high): eGFR ≥90 mL/min/1.73 m² 2
• G2 (Mildly decreased): eGFR 60-89 mL/min/1.73 m² 2
• G3a (Mildly to moderately decreased): eGFR 45-59 mL/min/1.73 m² 2
• G3b (Moderately to severely decreased): eGFR 30-44 mL/min/1.73 m² 2
• G4 (Severely decreased): eGFR 15-29 mL/min/1.73 m² 2
• G5 (Kidney failure): eGFR <15 mL/min/1.73 m² 2

Critical Point About G1 and G2

For stages G1 and G2, you cannot diagnose CKD based on eGFR alone—there must be evidence of kidney damage (albuminuria, imaging abnormalities, or biopsy findings) present for more than 3 months. 1, 2 This is a common pitfall: a young, healthy person with eGFR of 95 mL/min/1.73 m² and no albuminuria does not have CKD stage G1. 1

For stages G3-G5, an eGFR <60 mL/min/1.73 m² persisting for more than 3 months is sufficient to diagnose CKD, even without albuminuria. 1, 2

Albuminuria Categories (A Stages)

Albuminuria is measured using urine albumin-to-creatinine ratio (UACR) from a random spot urine sample: 1, 2

• A1 (Normal to mildly increased): UACR <30 mg/g creatinine 2
• A2 (Moderately increased): UACR 30-299 mg/g creatinine (formerly called "microalbuminuria") 2
• A3 (Severely increased): UACR ≥300 mg/g creatinine (formerly called "macroalbuminuria") 2

Complete CKD Classification Format

Always report CKD using all three components: cause, GFR category, and albuminuria category. 1 For example:

• "CKD G3a, A2 due to diabetes" 1
• "CKD G4, A3 due to hypertension" 1

Avoid outdated terminology like "mild, moderate, or severe CKD" or "CKD stage 1-5" without specifying the complete CGA classification. 1

Risk Stratification Matrix

The combination of GFR and albuminuria categories creates a color-coded risk matrix for CKD progression, cardiovascular events, and mortality: 2

• Green (Low risk): G1A1, G2A1 1, 2
• Yellow (Moderately high risk): G1A2, G2A2, G3aA1 1, 2
• Orange (High risk): G1A3, G2A3, G3aA2, G3bA1 1, 2
• Red (Very high risk): G3aA3, G3bA2, G3bA3, G4A1, G4A2, G4A3, G5A1, G5A2, G5A3 1, 2

At any given eGFR level, higher albuminuria increases risk substantially. 1 For example, a patient with G3aA3 has higher risk than a patient with G4A1, despite having better kidney function. 1

Measurement Methods

Estimating GFR

Use serum creatinine with the CKD-EPI 2009 equation to calculate eGFR for initial assessment. 3 The laboratory should automatically report eGFR when creatinine is measured. 3

If confirmation is needed due to extremes of muscle mass, diet, or assay interference, measure cystatin C and calculate eGFR using cystatin C-based equations (eGFRcys or eGFRcr-cys), or measure GFR directly using clearance methods. 3

Measuring Albuminuria

Collect a random spot urine sample for albumin and creatinine, and report the albumin-to-creatinine ratio (UACR). 1, 3 Timed or 24-hour collections are more burdensome and add little accuracy. 1

Avoid using urine albumin measurement alone without creatinine, as this is susceptible to false results due to hydration status. 1

Duration Requirement

Both kidney damage markers and reduced eGFR must persist for more than 3 months to diagnose CKD. 1, 4 This distinguishes CKD from acute kidney diseases and disorders (AKD), which last ≤3 months. 1

Terms to Avoid

Do not use these outdated terms: 1

• Chronic renal failure (CRF)
• End-stage renal disease (ESRD)
• Renal/kidney impairment, insufficiency, or dysfunction
• Mild, moderate, severe, early, or advanced CKD (without specifying G and A categories)
• Non-dialysis CKD (ND-CKD) or predialysis CKD

Nephrology Referral Thresholds

Refer to nephrology when: 1, 2

• eGFR <30 mL/min/1.73 m² (stages G4-G5) regardless of albuminuria 2
• Any stage with A3 albuminuria (≥300 mg/g) 2
• Rapidly declining eGFR or rapidly increasing albuminuria 1, 2
• Active urinary sediment (red cells, white cells, or cellular casts) 1
• Nephrotic syndrome 1
• Absence of retinopathy in type 1 diabetes with kidney disease 1