How is chronic kidney disease (CKD) classified in an adult patient?

CKD Classification in Adults

Chronic kidney disease must be classified using three dimensions: Cause (C), GFR category (G), and Albuminuria category (A)—the CGA classification system. 1

Diagnostic Criteria

CKD is defined as abnormalities of kidney structure or function present for ≥3 months with implications for health. 1, 2 Either of the following establishes the diagnosis:

• **GFR <60 mL/min/1.73 m²** (categories G3a-G5) persisting >3 months 3
• Evidence of kidney damage at any GFR level, most commonly albuminuria with ACR ≥30 mg/g 2, 3

GFR Categories (G)

The classification divides kidney function into six categories based on estimated GFR: 1, 3

• G1: ≥90 mL/min/1.73 m² (normal or high—requires evidence of kidney damage to diagnose CKD)
• G2: 60-89 mL/min/1.73 m² (mildly decreased—requires evidence of kidney damage to diagnose CKD)
• G3a: 45-59 mL/min/1.73 m² (mildly to moderately decreased)
• G3b: 30-44 mL/min/1.73 m² (moderately to severely decreased)
• G4: 15-29 mL/min/1.73 m² (severely decreased)
• G5: <15 mL/min/1.73 m² (kidney failure)

Critical distinction: The subdivision of G3 into G3a and G3b is essential because these categories have markedly different outcomes and risk profiles for mortality, cardiovascular disease, and progression to kidney failure. 1, 3

GFR Estimation Method

• Use the CKD-EPI equation for calculating creatinine-based eGFR, which has less bias than the MDRD equation, especially at eGFR ≥60 mL/min/1.73 m². 1, 2, 3
• For patients with eGFR 45-59 mL/min/1.73 m² (G3a) without albuminuria or other kidney damage markers, measure cystatin C to confirm CKD diagnosis, as this group represents 41% of persons estimated to have CKD based on creatinine alone. 1, 2, 3

Albuminuria Categories (A)

Albuminuria severity is classified into three categories using albumin-to-creatinine ratio (ACR): 1, 3

• A1: <30 mg/g (normal to mildly increased)
• A2: 30-300 mg/g (moderately increased)
• A3: >300 mg/g (severely increased)

Important: ACR ≥30 mg/g represents >3 times the normal value and is associated with increased risk for CKD complications. 1 Sex-specific cutoffs of >17 mg/g in men and >25 mg/g in women can also be used. 2

Cause Classification (C)

Assign cause based on: 1

• Presence or absence of systemic disease (e.g., diabetes, hypertension, autoimmune disease)
• Location within the kidney of observed or presumed pathologic-anatomic findings (glomerular, tubular, vascular, cystic)

Risk Stratification Using CGA

The combination of GFR and albuminuria categories determines prognosis for adverse outcomes including death, cardiovascular events, and kidney failure: 1

• Low risk: GFR ≥60 with ACR <30 mg/g
• Moderate risk: GFR ≥60 with ACR 30-300 mg/g OR GFR 45-59 with ACR <30 mg/g
• High risk: GFR ≥60 with ACR >300 mg/g OR GFR 45-59 with ACR 30-300 mg/g OR GFR 30-44 with ACR <30 mg/g
• Very high risk: GFR 45-59 or 30-44 with ACR >300 mg/g OR GFR 30-44 with ACR 30-300 mg/g OR GFR <30 with any albuminuria 1

Common Pitfalls

Avoid diagnosing CKD in patients with eGFR ≥60 mL/min/1.73 m² unless documented markers of kidney damage are present. 2 G1 and G2 categories alone do not fulfill CKD criteria without evidence of structural kidney damage. 1

Do not use creatinine clearance or estimated creatinine clearance, as these overestimate GFR and lead to misclassification. 2

Ensure abnormalities persist for ≥3 months before confirming CKD diagnosis to distinguish chronic from acute kidney disease. 1, 2