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

Classification of Chronic Kidney Disease Stages

CKD is classified using a comprehensive system that combines GFR categories (G1-G5) with albuminuria categories (A1-A3), creating a matrix that stratifies risk for disease progression, cardiovascular events, and mortality. 1

GFR Categories (G Stages)

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

• G1 (Normal or high): eGFR ≥90 mL/min/1.73 m² - requires evidence of kidney damage (albuminuria, imaging abnormalities, or biopsy findings) to diagnose CKD 1
• G2 (Mildly decreased): eGFR 60-89 mL/min/1.73 m² - also requires evidence of kidney damage to diagnose CKD 1
• G3a (Mildly to moderately decreased): eGFR 45-59 mL/min/1.73 m² 1
• G3b (Moderately to severely decreased): eGFR 30-44 mL/min/1.73 m² 1
• G4 (Severely decreased): eGFR 15-29 mL/min/1.73 m² 1
• G5 (Kidney failure): eGFR <15 mL/min/1.73 m² 1

Albuminuria Categories (A Stages)

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

• A1 (Normal to mildly increased): UACR <30 mg/g creatinine (or <3 mg/mmol) 1
• A2 (Moderately increased): UACR 30-299 mg/g creatinine (or 3-29 mg/mmol) 1
• A3 (Severely increased): UACR ≥300 mg/g creatinine (or ≥30 mg/mmol) 1

Complete CKD Classification

The complete classification requires specifying cause (C), GFR category (G), and albuminuria category (A). For example: "CKD G3a, A2 due to diabetes" or "CKD G4, A3 due to hypertension." 1, 2

Risk Stratification Using the CGA Matrix

The combination of GFR and albuminuria categories creates a color-coded risk matrix for CKD progression and adverse outcomes: 1, 2

• Green (Low risk): G1A1, G2A1 - annual monitoring 1, 2
• Yellow (Moderately high risk): G1A2, G2A2, G3aA1 - monitoring at least once yearly 1
• Orange (High risk): G1A3, G2A3, G3aA2, G3bA1 - monitoring twice yearly 1
• Red (Very high risk): G3aA3, G3bA2, G3bA3, G4A1, G4A2, G4A3 - monitoring three times yearly 1
• Dark red (Highest risk): G5A1, G5A2, G5A3 - monitoring four times yearly 1

Critical Diagnostic Requirements

For stages G1 and G2, CKD cannot be diagnosed by eGFR alone - there must be evidence of kidney damage (albuminuria, imaging abnormalities, or biopsy findings) present for >3 months. 1, 3

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

Confirmation of Albuminuria

Two of three UACR specimens collected within a 3-6 month period should be abnormal before confirming high albuminuria, due to biological variability exceeding 20% between measurements. 1, 2

Nephrology Referral Thresholds

Refer to nephrology when: 1

• eGFR <30 mL/min/1.73 m² (stages G4-G5) regardless of albuminuria 1
• Any stage with A3 albuminuria (≥300 mg/g) 1, 2
• Rapidly declining eGFR or rapidly increasing albuminuria 1
• Active urinary sediment, nephrotic syndrome, or absence of retinopathy in type 1 diabetes 1

Common Pitfalls

The eGFR must be calculated using validated equations - the CKD-EPI equation is preferred over the older MDRD equation as it provides better accuracy, especially at higher GFR levels. 1, 4, 5

Avoid diagnosing CKD based on a single measurement - both eGFR <60 mL/min/1.73 m² and albuminuria must persist for >3 months to meet diagnostic criteria. 1, 3

Do not use descriptive terms like "mild," "moderate," or "severe" CKD - always specify the complete CGA classification (cause, GFR category, albuminuria category) for accurate risk stratification and treatment planning. 1