What is the complete KDIGO staging classification for chronic kidney disease?

KDIGO Staging of Chronic Kidney Disease

The KDIGO classification system stages CKD using a three-dimensional framework: Cause (C), GFR category (G1–G5), and Albuminuria category (A1–A3), requiring all abnormalities to persist for at least 3 months to distinguish chronic from acute kidney disease. 1

GFR Categories (G Stages)

KDIGO divides estimated glomerular filtration rate into six categories: 1

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

Stage 3 is subdivided into 3a and 3b because these GFR ranges demonstrate markedly different mortality, cardiovascular risk, and CKD progression profiles. 1 The subdivision is driven by outcome data showing substantially greater risk in the 30–44 range compared to 45–59. 2

For patients receiving dialysis, use the G5D designation to distinguish them from non-dialyzed G5 patients, as this distinction is critical for treatment and prognosis. 2, 1 Similarly, use a "T" suffix for kidney transplant recipients. 2, 1

Albuminuria Categories (A Stages)

KDIGO classifies albumin-to-creatinine ratio (ACR) into three categories: 1

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

Measure ACR on a random spot urine sample rather than 24-hour collection, as spot samples are less burdensome and equally predictive. 2 The 30 mg/g threshold represents approximately 3 times the normal value in young adults and independently predicts CKD complications, cardiovascular mortality, and progression to kidney failure. 1

Confirm albuminuria by documenting abnormal values on at least 2 of 3 specimens collected over 3–6 months before diagnosing persistent albuminuria, due to biological variability in urinary albumin excretion. 2

Critical Diagnostic Requirements

CKD can be diagnosed in G1–G2 ONLY when evidence of kidney damage is documented—GFR alone is insufficient at these stages. 1 Evidence of kidney damage includes: 1

• Albuminuria (ACR ≥30 mg/g)
• Abnormal urine sediment
• Electrolyte disturbances from tubular dysfunction
• Structural abnormalities on imaging
• Histologic abnormalities on biopsy
• History of kidney transplantation

For G3–G5, CKD can be diagnosed based on reduced GFR alone, with or without evidence of kidney damage. 2, 1

All abnormalities must persist for at least 3 months to meet CKD criteria and distinguish chronic from acute kidney disease. 1, 3 This temporal requirement is non-negotiable. 4

Combined Risk Stratification (KDIGO Heat Map)

KDIGO integrates GFR and albuminuria categories into a color-coded risk matrix that guides monitoring frequency, blood pressure targets, nephrology referral timing, and cardiovascular risk-reduction strategies: 1

Low Risk (Green)

• G1–G2 with A1
• If no other markers of kidney disease are present, CKD is NOT diagnosed 1

Moderately Increased Risk (Yellow)

• G1–G2 with A2
• G3a with A1 1

High Risk (Orange)

• G1–G2 with A3
• G3a with A2
• G3b with A1 1

Very High Risk (Red)

• G3a with A3
• G3b with A2 or A3
• Any G4–G5 (regardless of albuminuria level) 1

Patients in the very high risk category have the highest risk of CKD progression, cardiovascular events, and mortality, requiring nephrology referral and intensive management. 1

GFR Estimation Method

Use the CKD-EPI creatinine equation as the first-line method for eGFR calculation because it demonstrates less bias than the MDRD equation, particularly when eGFR ≥60 mL/min/1.73 m². 1, 5 The CKD-EPI equation shows improved precision and greater accuracy across the GFR spectrum. 1

When creatinine-based eGFR is uncertain or when precise GFR measurement affects clinical decisions, confirm with cystatin C-based or combined creatinine-cystatin C equations. 1, 5 The 2024 KDIGO guidelines emphasize preferential use of combined equations when cystatin C is available. 5

Ensure creatinine assays are calibrated to isotope-dilution mass spectrometry (IDMS) reference standards to minimize systematic bias. 1

Complete CKD Classification Notation

Never stage CKD by GFR alone—always report the full CGA notation to ensure accurate risk stratification and appropriate management. 1 For example: "CKD G3b A2 due to diabetes" provides complete prognostic information, whereas "Stage 3 CKD" does not. 2, 1

The cause (C) component is assigned based on systemic disease presence and anatomic location of pathology (glomerular, tubular, vascular, cystic). 1 Common causes include diabetic kidney disease, hypertensive nephrosclerosis, glomerulonephritis, and polycystic kidney disease. 1

Management Focus by CKD Stage

The stage-specific management priorities are: 2

• G1–G2: Diagnose cause, evaluate and treat risk factors for CKD progression (hypertension, hyperglycemia, albuminuria)
• G3: Continue risk factor management, evaluate and treat CKD complications (anemia, bone disease, metabolic acidosis)
• G4: Intensive complication management, prepare for kidney replacement therapy
• G5: Kidney replacement therapy (dialysis or transplantation) if uremic symptoms develop

As GFR decreases below 60 mL/min/1.73 m², the risk of complications such as hypertension, anemia, and hyperphosphatemia increases significantly. 1 Hypertension prevalence approaches 80% in G4 CKD. 1

Common Pitfalls to Avoid

Do not diagnose CKD based on a single abnormal test—the abnormality must be documented on at least two occasions ≥3 months apart to satisfy the chronicity criterion. 1, 3 This prevents misclassification of acute kidney injury as CKD. 6

Do not use back-calculated eGFR of 75 mL/min/1.73 m² to impute baseline creatinine, as this overestimates AKI incidence in populations with high CKD prevalence. 6 Always search aggressively for prior creatinine values in medical records. 6

Do not rely on serum creatinine alone without measuring albuminuria, as both provide independent prognostic information for cardiovascular events, CKD progression, and mortality. 7 There is no correlation between GFR and albuminuria progression, so both parameters must be monitored independently. 1