Classification of Chronic Kidney Disease Based on GFR and Albuminuria
Chronic Kidney Disease (CKD) is classified using a comprehensive system based on Glomerular Filtration Rate (GFR) categories (G1-G5) and albuminuria categories (A1-A3), which provides precise risk stratification for disease progression, mortality, and complications.
GFR Categories
- G1: Normal or high GFR (≥90 mL/min/1.73 m²)
- G2: Mildly decreased GFR (60-89 mL/min/1.73 m²)
- G3a: Mildly to moderately decreased GFR (45-59 mL/min/1.73 m²)
- G3b: Moderately to severely decreased GFR (30-44 mL/min/1.73 m²)
- G4: Severely decreased GFR (15-29 mL/min/1.73 m²)
- G5: Kidney failure (<15 mL/min/1.73 m²)
Important note: In the absence of evidence of kidney damage, neither GFR category G1 nor G2 fulfill the criteria for CKD 1.
Albuminuria Categories
- A1: Normal to mildly increased albuminuria (<30 mg/g creatinine)
- A2: Moderately increased albuminuria (30-299 mg/g creatinine)
- A3: Severely increased albuminuria (≥300 mg/g creatinine)
Urine reagent strip results can be substituted when albuminuria measurement is not available 1.
Combined Classification System
The combination of GFR and albuminuria categories provides a more precise evaluation of CKD risk than either parameter alone 2. This combined approach is referred to as the CGA system (Cause, GFR category, and Albuminuria category) 1.
The risk stratification based on this combined approach is as follows:
- Low risk: G1A1, G2A1 (green) - Note: These require other evidence of kidney damage to qualify as CKD
- Moderately increased risk: G1A2, G2A2, G3aA1 (yellow)
- High risk: G1A3, G2A3, G3aA2, G3bA1 (orange)
- Very high risk: G3aA3, G3bA2-A3, G4A1-A3, G5A1-A3 (red)
Clinical Implications and Monitoring
The frequency of monitoring should be adjusted according to the risk category 2:
- Low risk: Annual monitoring
- Moderate risk: Every 6-12 months
- High risk: Every 3-6 months
- Very high risk: Every 1-3 months
Referral to Nephrology
Referral to a nephrologist is recommended for:
- Patients with GFR <30 mL/min/1.73 m² (G4-G5)
- Patients with severely increased albuminuria (A3)
- Patients with rapid decline in GFR
- Uncertainty about the cause of kidney disease 1
Important Clinical Considerations
The distinction between CKD stage 3a and 3b is clinically significant, as the risks of mortality and other adverse outcomes vary greatly between these groups 1.
Albuminuria is strongly predictive of outcomes at all levels of GFR, both at individual and population levels 1, 3.
Lower GFR is strongly associated with higher odds of multiple laboratory abnormalities (anemia, acidosis, hyperphosphatemia, etc.), whereas albuminuria has weaker associations with these concurrent complications 3.
Even within the normoalbuminuric range (<30 mg/g), higher levels of albuminuria are associated with increased risk of CKD progression 4.
The CKD-EPI equation is currently recommended as the most accurate for estimating GFR in routine clinical practice 2.
This classification system provides a comprehensive framework for assessing risk, guiding monitoring frequency, and determining appropriate referral timing for patients with CKD, ultimately improving outcomes related to morbidity, mortality, and quality of life.