Chronic Kidney Disease Classification
CKD is classified based on three components: Cause, GFR category (G1-G5), and Albuminuria category (A1-A3), with abnormalities present for at least 3 months. 1
Core Definition
CKD is defined as abnormalities of kidney structure or function present for more than 3 months with implications for health. 1 This temporal requirement distinguishes chronic from acute kidney disease and must be confirmed before establishing a CKD diagnosis. 1, 2
The diagnosis relies on either:
- Markers of kidney damage (albuminuria, proteinuria, structural abnormalities on imaging, or abnormal urine sediment), OR 1
- Reduced GFR (<60 mL/min/1.73 m²) 1
GFR-Based Staging System
The classification uses five GFR stages (G1-G5): 1, 2
| Stage | GFR (mL/min/1.73 m²) | Description |
|---|---|---|
| G1 | ≥90 | Kidney damage with normal or increased GFR* |
| G2 | 60-89 | Kidney damage with mild decrease in GFR* |
| G3a | 45-59 | Moderate decrease in GFR |
| G3b | 30-44 | Moderate to severe decrease in GFR |
| G4 | 15-29 | Severe decrease in GFR |
| G5 | <15 | Kidney failure |
*For stages G1 and G2, evidence of kidney damage (such as albuminuria) is required for CKD diagnosis, as GFR alone is insufficient at these levels. 1, 2 Stages G3-G5 can be diagnosed based on GFR alone. 2
Stage 3 Subdivision
Stage 3 is subdivided into 3a (GFR 45-59) and 3b (GFR 30-44) because mortality and cardiovascular risks vary significantly between these ranges. 1 This distinction has broad clinical applications given the high prevalence of Stage 3 CKD. 1
Albuminuria Categories
Albuminuria must be integrated into the classification for complete risk stratification: 1, 2
| Category | Albumin-to-Creatinine Ratio | Description |
|---|---|---|
| A1 | <30 mg/g | Normal to mildly increased |
| A2 | 30-299 mg/g | Moderately increased |
| A3 | ≥300 mg/g | Severely increased |
An albuminuria level of 30 mg/g represents more than 3 times the normal value in young adults and independently predicts increased risk for CKD complications, cardiovascular mortality, and progression to kidney failure. 2 When albuminuria measurement is unavailable, urine reagent strip results can be substituted. 1
Complete CGA Classification
The complete classification requires all three components: Cause (C), GFR category (G), and Albuminuria category (A). 1 Classifying by GFR alone is incomplete and fails to capture the full risk profile. 2
Cause Assignment
Assign cause based on: 1
- Presence or absence of systemic disease
- Location within the kidney of observed or presumed pathologic-anatomic findings
Treatment Modifiers
Add suffix "T" for all kidney transplant recipients at any GFR level, and "D" for Stage 5 patients treated by dialysis. 1
GFR Estimation Method
Use the CKD-EPI equation for estimating GFR in adults from serum creatinine calibrated to isotope-dilution mass spectrometry reference method. 2 The CKD-EPI equation demonstrates less bias than the MDRD equation, especially at GFR ≥60 mL/min/1.73 m², with improved precision and greater accuracy. 2
Do not rely on serum creatinine alone—use prediction equations that account for age, sex, race, and body size. 2, 3
Clinical Implications by Stage
Stages 1-2 (GFR ≥60)
- Focus on early detection, CKD risk reduction, and treating comorbid conditions 2, 3
- Requires evidence of kidney damage for diagnosis 2
Stage 3 (GFR 30-59)
- Estimate progression rate and evaluate for complications 2
- Risk of complications (hypertension, anemia, hyperphosphatemia) increases significantly below GFR 60 2, 3
- Annual monitoring minimum for Stage 3a without elevated albuminuria 4
Stage 4 (GFR 15-29)
- Intensive management of complications 2
- Prepare for kidney replacement therapy 3
- Hypertension prevalence approaches 80% 2
- Refer to nephrology for consultation and co-management 3
Stage 5 (GFR <15)
Critical Pitfalls to Avoid
- Never classify using GFR alone—always include albuminuria category for complete risk stratification 2
- Do not diagnose CKD from a single measurement—abnormalities must persist for at least 3 months 1, 2, 4
- Do not assume normal aging explains reduced GFR—decreased GFR in elderly remains an independent predictor of adverse outcomes 3
- For Stages 1-2, do not diagnose CKD without evidence of kidney damage (albuminuria, proteinuria, or structural abnormalities) 1, 2
- Adjust medication doses at GFR <60, particularly for renally cleared drugs 4
Risk Stratification
Combining GFR and albuminuria categories creates a risk matrix (low, moderately high, high, very high risk) that guides monitoring frequency and treatment intensity. 2 For example, a patient with GFR 47 (G3a) and normal albuminuria (A1) is moderate risk requiring annual monitoring, but if albuminuria increases to A2 or A3, risk escalates substantially requiring monitoring 2-3 times yearly and consideration for nephrology referral. 4