How is chronic kidney disease classified?

How Chronic Kidney Disease is Classified

Chronic kidney disease must be classified using the complete CGA system: Cause of disease, GFR category (G1-G5), and Albuminuria category (A1-A3), not by GFR alone. 1, 2

Core Diagnostic Requirements

Before classification can occur, CKD must first be diagnosed by documenting that kidney abnormalities persist for more than 3 months to distinguish chronic from acute disease. 1, 3 This requires:

• Either eGFR <60 mL/min/1.73 m² on at least two measurements ≥3 months apart 3, 4
• Or evidence of kidney damage (albuminuria ≥30 mg/g, abnormal imaging, abnormal biopsy, abnormal sediment) present on repeat testing over 3 months 1, 4

The Three-Component CGA Classification System

1. Cause (C)

Assign the underlying cause based on presence or absence of systemic disease and the anatomic location of kidney pathology (glomerular, tubular, vascular, or cystic). 1 Common causes include diabetic kidney disease, hypertensive nephrosclerosis, glomerulonephritis, and polycystic kidney disease. 2

2. GFR Categories (G1-G5)

The GFR staging divides kidney function into six categories: 1, 2

• G1: eGFR ≥90 mL/min/1.73 m² (normal or high function)
• G2: eGFR 60-89 mL/min/1.73 m² (mildly decreased)
• G3a: eGFR 45-59 mL/min/1.73 m² (mild-to-moderate decrease)
• G3b: eGFR 30-44 mL/min/1.73 m² (moderate-to-severe decrease)
• G4: eGFR 15-29 mL/min/1.73 m² (severe decrease)
• G5: eGFR <15 mL/min/1.73 m² (kidney failure)

Critical distinction: For G1 and G2, CKD cannot be diagnosed by GFR alone—you must document evidence of kidney damage such as albuminuria, abnormal imaging, or abnormal sediment. 1, 2, 4 An otherwise healthy person with eGFR 55 mL/min/1.73 m² and no other abnormalities does not have CKD. 1

The subdivision of G3 into G3a and G3b is mandatory because these ranges have significantly different mortality, cardiovascular risk, and progression profiles. 1, 2, 4

Add suffix "D" for dialysis (G5D) and "T" for transplant recipients to distinguish these populations requiring specialized care. 1, 2

3. Albuminuria Categories (A1-A3)

Measure albumin-to-creatinine ratio (ACR) on a random spot urine sample: 1, 2, 3

• A1: ACR <30 mg/g (normal to mildly increased)
• A2: ACR 30-300 mg/g (moderately increased; formerly "microalbuminuria")
• A3: ACR >300 mg/g (severely increased; includes nephrotic-range proteinuria)

Confirm albuminuria with 2 of 3 specimens over 3-6 months before finalizing the A category. 3 The 30 mg/g threshold represents more than 3 times the normal value and independently predicts CKD complications, cardiovascular mortality, and progression to kidney failure. 2, 4

Combined Risk Stratification (The KDIGO Heat Map)

The combination of GFR and albuminuria categories creates a color-coded prognostic matrix: 1, 2, 3

• Low risk (green): G1-G2 with A1 only
• Moderately increased risk (yellow): G1-G2 with A2, or G3a with A1
• High risk (orange): G1-G2 with A3, G3a with A2, or G3b with A1
• Very high risk (red): G3a with A3, G3b with A2-A3, or any G4-G5 regardless of albuminuria

This risk stratification drives clinical decisions including frequency of monitoring, intensity of blood pressure control, nephrology referral timing, and cardiovascular risk management. 2, 3

GFR Estimation Method

Use the CKD-EPI creatinine equation for initial eGFR calculation because it has less bias than the older MDRD equation, especially at eGFR ≥60 mL/min/1.73 m². 1, 2, 4 Creatinine assays must be calibrated to isotope-dilution mass spectrometry (IDMS) reference standards. 2, 4

When creatinine-based eGFR is uncertain or clinical decisions require precision, confirm with cystatin C-based or combined creatinine-cystatin C equations. 2 This is particularly important for patients with eGFR 45-59 mL/min/1.73 m² (G3a) without albuminuria, where disease labeling is controversial. 4

Common Pitfalls to Avoid

Never classify CKD by GFR stage alone—always include cause and albuminuria category for complete staging (e.g., "CKD G3b, A2 due to diabetes" not just "stage 3 CKD"). 1, 2 Incomplete classification leads to inadequate risk stratification and inappropriate management decisions.

Do not diagnose CKD based on a single abnormal test—the 3-month persistence requirement is mandatory to exclude acute kidney injury or transient abnormalities. 1, 3, 4

Do not overlook albuminuria testing—59% of patients with CKD in one large study lacked albuminuria assessment, and those without testing had 53% lower odds of receiving guideline-recommended nephrology care despite meeting eGFR criteria for referral. 5 Both GFR and albuminuria must be monitored independently as they do not correlate with each other. 2

Recognize that GFR estimation formulas underestimate true GFR when kidney function is subnormal (GFR <90 mL/min/1.73 m²), so serial measurements over time are essential to assess progression rather than relying on single values. 2