How is Chronic Kidney Disease (CKD) staged based on glomerular filtration rate (GFR) and albuminuria?

Staging Chronic Kidney Disease

CKD is staged using a comprehensive CGA classification system that combines GFR categories (G1-G5), albuminuria categories (A1-A3), and cause of disease to stratify risk for progression, cardiovascular events, and mortality. 1, 2

Core Diagnostic Requirements

CKD requires abnormalities present for >3 months to distinguish chronic from acute disease. 1, 3 This chronicity must be documented through:

• Repeat eGFR measurements at least 3 months apart 3
• Two of three UACR specimens abnormal within 3-6 months 1, 3
• Historical laboratory data or imaging findings 4

GFR Categories (G Stages)

The five GFR stages are defined by estimated glomerular filtration rate in mL/min/1.73 m²: 1, 2

• G1 (Normal or high): eGFR ≥90
• G2 (Mildly decreased): eGFR 60-89
• G3a (Mildly to moderately decreased): eGFR 45-59
• G3b (Moderately to severely decreased): eGFR 30-44
• G4 (Severely decreased): eGFR 15-29
• G5 (Kidney failure): eGFR <15

Critical Caveat for G1 and G2

In the absence of evidence of kidney damage, neither G1 nor G2 fulfill criteria for CKD. 1, 2, 3 For stages G1 and G2, you must document kidney damage (albuminuria ≥30 mg/g, imaging abnormalities, or biopsy findings) present for >3 months. 2, 3 An eGFR of 60-89 mL/min/1.73 m² with normal UACR and no other kidney damage markers does not constitute CKD. 4

Stages G3-G5

For stages G3-G5, an eGFR <60 mL/min/1.73 m² persisting for >3 months is sufficient to diagnose CKD, even without albuminuria. 2, 5

Albuminuria Categories (A Stages)

Albuminuria is measured using urine albumin-to-creatinine ratio (UACR) from a random spot urine sample: 1, 2

• A1 (Normal to mildly increased): UACR <30 mg/g
• A2 (Moderately increased): UACR 30-299 mg/g
• A3 (Severely increased): UACR ≥300 mg/g

At least 2 of 3 specimens should be abnormal to confirm albuminuria due to biological variability. 1, 3

Risk Stratification Matrix

The combination of GFR and albuminuria creates a color-coded risk matrix for CKD progression and adverse outcomes: 2

• Green (Low risk): G1A1, G2A1
• Yellow (Moderately high risk): G1A2, G2A2, G3aA1
• Orange (High risk): G1A3, G2A3, G3aA2, G3bA1
• Red (Very high risk): G3aA3, G3bA2, G3bA3, G4A1, G4A2, G4A3
• Dark red (Highest risk): G5A1, G5A2, G5A3

Both lower eGFR and higher albuminuria independently predict mortality and end-stage renal disease, with associations stronger for ESRD than mortality. 6, 7 Lower eGFR is more strongly associated with CKD complications (anemia, acidosis, hyperphosphatemia, hypertension) than albuminuria after controlling for GFR. 8

Laboratory Testing Approach

For initial GFR assessment, measure serum creatinine and report eGFR using the CKD-EPI equation. 9

If confirmation is needed due to extremes of muscle mass, diet, or assay interference, measure cystatin C and calculate eGFR using both creatinine and cystatin C (eGFRcr-cys). 3, 9 When creatinine-based eGFR is 45-59 mL/min/1.73 m² without albuminuria, measuring cystatin C is recommended because persons with both eGFR creatinine and cystatin C >60 mL/min/1.73 m² have very low risk and could be considered not to have CKD. 3

For albuminuria assessment, measure UACR in a random spot urine collection. 1, 9 Timed or 24-hour collections are more burdensome and add little accuracy. 1 If confirmation is required due to diurnal variation or extremes of muscle mass/diet, measure albumin excretion rate from a timed collection. 9

Common Pitfalls to Avoid

The most critical error is diagnosing CKD based on a single eGFR measurement or mild eGFR reduction without confirming chronicity or documenting kidney damage. 3, 4 eGFR can fluctuate by >20% between measurements even in stable individuals. 3

Never diagnose Stage 2 CKD based on eGFR alone - it absolutely requires documented kidney damage in addition to mildly decreased eGFR. 4 A single eGFR measurement could represent acute kidney injury rather than chronic disease. 4

Nephrology Referral Thresholds

Refer to nephrology when: 2

• eGFR <30 mL/min/1.73 m² (stages G4-G5) regardless of albuminuria
• Any stage with A3 albuminuria (≥300 mg/g)
• Rapidly declining eGFR or rapidly increasing albuminuria
• Active urinary sediment or nephrotic syndrome
• Absence of retinopathy in type 1 diabetes

Surveillance Strategy

Both albuminuria and eGFR should be monitored annually to enable timely diagnosis, monitor progression, detect superimposed kidney diseases including acute kidney injury, assess risk of complications, dose medications appropriately, and determine nephrology referral needs. 1 When eGFR is <60 mL/min/1.73 m², screening for CKD complications is indicated. 1