CKD Classification, Staging, and Progression

Chronic kidney disease is classified along three axes: the underlying cause, the level of kidney function expressed as a glomerular filtration rate (GFR) category, and the degree of albuminuria. This Cause-GFR-Albuminuria (CGA) scheme replaced earlier purely GFR-based staging and is the framework through which the disease is described, risk-stratified, and followed for progression.

Definition

CKD classification assigns a patient to a cause, a GFR category (G1 >=90, G2 60-89, G3a 45-59, G3b 30-44, G4 15-29, G5 <15 mL/min/1.73 m2), and an albuminuria category (A1 <30, A2 30-300, A3 >300 mg/g), with the combination predicting the risk of adverse outcomes.

Scope

The topic covers how kidney function is measured and estimated, the GFR categories G1 through G5, the albuminuria categories A1 through A3, the combined CGA classification and its risk-stratifying "heat map," and what counts as progression. It is a reference account of the classification system and the evidence behind it, not a protocol for managing an individual patient.

Core questions

• How is glomerular filtration rate measured and estimated?
• What are the GFR and albuminuria categories and how are they combined?
• What defines progression of CKD as opposed to stable disease?
• Why was albuminuria added to a previously function-based staging system?

Key concepts

• Glomerular filtration rate (GFR)
• Estimated GFR from creatinine and cystatin C
• GFR categories G1-G5
• Albuminuria categories A1-A3
• Cause-GFR-Albuminuria (CGA) classification
• KDIGO risk heat map
• Rapid progression and GFR slope

Mechanisms

GFR is the volume of plasma cleared of a filtration marker per unit time and is the best overall index of kidney function; in practice it is estimated from serum creatinine or cystatin C using validated equations rather than measured directly. Albuminuria reflects loss of glomerular barrier integrity and tubular handling and is both a marker of damage and an independent driver of progression. The CGA scheme combines these because pooled cohort data showed that lower GFR and higher albuminuria each contribute independently and multiplicatively to the risks of death, cardiovascular events, and kidney failure; progression is recognised as movement to a higher-risk category with a sustained fall in GFR, or as a consistently steep decline in GFR slope over time.

Clinical relevance

The CGA classification is the shared language used to describe where a person sits on the spectrum of kidney disease and how their risk is expected to change, which is why estimated GFR and albuminuria are reported together. As a reference topic it explains how staging is constructed and validated; it characterises risk categories and does not prescribe testing intervals or treatment for any individual.

Epidemiology

Most people with CKD are in the earlier GFR categories (G1-G3) and are often asymptomatic, so the condition is frequently identified through laboratory testing rather than symptoms. The graded association of GFR and albuminuria with mortality and kidney failure, demonstrated across large international cohort collaborations, underpins the validity of the staging system.

History

The National Kidney Foundation's KDOQI guideline of 2002 first defined CKD and staged it by GFR alone. The KDIGO 2012 guideline revised this into the CGA framework, formally incorporating albuminuria after collaborative meta-analyses showed it added independent prognostic information, and the KDIGO 2024 update retained and consolidated this classification.

Debates

Should estimating equations include a race coefficient?

Earlier creatinine-based GFR equations used a race term that was later judged to introduce inequity, prompting a move toward race-free estimating equations and greater use of cystatin C; how best to estimate GFR fairly and accurately remains an active methodological discussion.

Key figures

• Andrew S. Levey
• Josef Coresh
• Kai-Uwe Eckardt

Seminal works

• kdigo-2012-ckd
• levey-2010
• stevens-2024-kdigo

Frequently asked questions

What do the letters and numbers in a CKD stage such as G3a A2 mean?

The G value gives the GFR category (here 45-59 mL/min/1.73 m2) and the A value gives the albuminuria category (here 30-300 mg/g); together with the cause they place the person on the KDIGO risk map.

Why is albuminuria part of staging and not just GFR?

Pooled cohort data showed that albuminuria predicts death, cardiovascular events, and kidney failure independently of GFR, so combining the two gives a more accurate picture of risk than function alone.