CKD Staging Using KDIGO Guidelines and eGFR
Chronic Kidney Disease is staged using a comprehensive system that combines eGFR categories (G1-G5) with albuminuria categories (A1-A3), creating a heat map that stratifies risk for disease progression, cardiovascular disease, and mortality. 1
Core Definition of CKD
CKD is defined as kidney damage or eGFR <60 mL/min/1.73 m² persisting for 3 months or more, irrespective of cause 2. The diagnosis requires either:
- eGFR <60 mL/min/1.73 m² (stages G3-G5), OR
- Evidence of kidney damage (typically albuminuria ≥30 mg/g creatinine) with any eGFR level 1
- Other manifestations such as abnormal urinary sediment, radiographic abnormalities, or pathological findings 1
eGFR-Based Staging (G Categories)
The KDIGO classification divides kidney function into six categories based on eGFR 1:
- G1: eGFR ≥90 mL/min/1.73 m² (normal or high) - requires evidence of kidney damage for CKD diagnosis
- G2: eGFR 60-89 mL/min/1.73 m² (mildly decreased) - requires evidence of kidney damage for CKD diagnosis
- G3a: eGFR 45-59 mL/min/1.73 m² (mildly to moderately decreased)
- G3b: eGFR 30-44 mL/min/1.73 m² (moderately to severely decreased)
- G4: eGFR 15-29 mL/min/1.73 m² (severely decreased)
- G5: eGFR <15 mL/min/1.73 m² (kidney failure)
Critical distinction: Stages G1 and G2 require documented evidence of kidney damage (usually albuminuria) to diagnose CKD, while stages G3-G5 are defined by reduced eGFR alone, with or without kidney damage 1.
Albuminuria-Based Staging (A Categories)
Albuminuria is measured using urine albumin-to-creatinine ratio (UACR) in a random spot urine collection 1:
- A1: UACR <30 mg/g (normal to mildly increased)
- A2: UACR 30-300 mg/g (moderately increased)
- A3: UACR >300 mg/g (severely increased)
Important caveat: Due to biological variability exceeding 20%, two of three specimens collected within 3-6 months should be abnormal before confirming persistent albuminuria 1.
Recommended eGFR Calculation Methods
Use creatinine-based eGFR (eGFRcr) as the initial assessment method 1. The CKD-EPI equation is generally preferred over the MDRD equation 1.
When cystatin C is available, use the combined creatinine-cystatin C equation (eGFRcr-cys) for more accurate GFR staging, particularly in adults at risk for CKD (Grade 1B recommendation) 1. The newer EKFC equations are particularly suited for European populations and applicable across all age groups 3.
Establishing Chronicity
Proof of chronicity (≥3 months duration) can be established through 1:
- Review of past eGFR measurements
- Review of past albuminuria or proteinuria measurements
- Imaging findings (reduced kidney size, cortical thinning)
- Kidney biopsy showing fibrosis/atrophy
- Medical history of conditions causing CKD
- Repeat measurements within and beyond 3 months
Do not assume chronicity from a single abnormal eGFR or UACR, as this could represent acute kidney injury or acute kidney disease 1.
Risk Stratification and Clinical Implications
The KDIGO system creates a risk stratification matrix combining eGFR and albuminuria categories, with risk increasing from green (low) to yellow (moderate) to orange (high) to red (very high) for adverse outcomes 1, 4.
Nephrology referral thresholds 1:
- eGFR 30-60 mL/min/1.73 m²: Transition from primary care to nephrology care
- eGFR <30 mL/min/1.73 m²: Transition to interprofessional care
- Risk-based criteria: 5-year kidney failure risk ≥3-5% warrants nephrology referral 1
Access and transplant planning should begin when 1:
- eGFR <20 mL/min/1.73 m², OR
- 2-year kidney failure risk ≥40%
Monitoring Frequency
Both eGFR and albuminuria should be monitored at least annually to detect CKD, monitor progression, detect superimposed kidney diseases including acute kidney injury, and guide medication dosing 1. The frequency should increase with CKD severity 1.
Common Pitfalls to Avoid
Age considerations: The CKD-EPI equation produces higher eGFR estimates in younger individuals and lower estimates in the elderly compared to MDRD 5. Consider age-dependent threshold values when interpreting results in older adults 5.
Single measurements: Never diagnose CKD based on a single abnormal test result due to biological variability and potential for acute processes 1.
Ignoring albuminuria: At any eGFR level, the degree of albuminuria independently predicts risk of CKD progression, cardiovascular disease, and mortality 1. Both parameters must be assessed for complete risk stratification 1.