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Here is a comprehensive, exam-focused overview of CKD — its definition, classification, and staging per current KDIGO guidelines.

Chronic Kidney Disease (CKD) — Overview, Classification & Staging

Definition

CKD is defined as abnormalities of kidney structure or function, present for >3 months, with implications for health. Two criteria can each independently satisfy the diagnosis:

Criterion	Threshold
Decreased GFR	eGFR < 60 mL/min/1.73 m²
Markers of kidney damage	Albuminuria (ACR ≥30 mg/g), abnormal urine sediment, electrolyte abnormalities, structural abnormalities on imaging, history of kidney transplantation

Key exam point: G1 or G2 with A1 (eGFR ≥60 and ACR <30 mg/g) does not meet CKD criteria unless there is additional evidence of kidney damage. The 3-month duration requirement distinguishes CKD from AKI.

— Brenner and Rector's The Kidney, Comprehensive Clinical Nephrology 7th Ed.

KDIGO CGA Classification System (2012)

The current system uses 3 dimensions: Cause + GFR category + Albuminuria category.

G Stages — GFR Categories

Stage	Description	eGFR (mL/min/1.73 m²)
G1	Normal or high	≥90
G2	Mildly decreased	60–89
G3a	Mildly to moderately decreased	45–59
G3b	Moderately to severely decreased	30–44
G4	Severely decreased	15–29
G5	Kidney failure	<15 (or on KRT)

A Stages — Albuminuria Categories (urine ACR)

Stage	Description	ACR (mg/g)	ACR (mg/mmol)
A1	Normal to mildly increased	<30	<3
A2	Moderately increased	30–300	3–30
A3	Severely increased	>300	>30

KDIGO Heat Map — CGA Classification Diagram

KDIGO CKD Classification 3D Cube showing G stages (GFR), A stages (albuminuria), and cause categories

The color-coded cube (green → yellow → orange → red) reflects increasing risk of progression to kidney failure as GFR falls and albuminuria rises.

Monitoring Frequency by Stage

Numbers in the heat map below indicate recommended nephrology follow-up visits per year:

KDIGO monitoring frequency heat map — numbers represent annual nephrology visits per GFR/albuminuria category

G5/A3 patients require 4+ visits per year. G1–G2/A1 with confirmed CKD require 1 visit/year.

GFR Estimation Equations

Two main equations to know for exams:

Equation	Formula	Notes
CKD-EPI	Uses creatinine ± cystatin C, age, sex	Most accurate; preferred in North America
Cockcroft-Gault	(140 − Age) × Weight / (72 × SCr) × 0.85 (if female)	Estimates CrCl, not GFR; used for drug dosing
MDRD	1.86 × (SCr)^−1.154 × (Age)^−0.203	Older; underestimates at higher GFR values

Cause Classification ("C" in CGA)

KDIGO categorizes cause by:

Presence/absence of systemic disease (e.g., diabetes, lupus, amyloid)
Location of pathologic abnormality within the kidney:
- Glomerular diseases
- Tubulointerstitial diseases
- Vascular diseases
- Cystic & congenital diseases

Most Common Causes (Adults)

Diabetic nephropathy — #1 cause of ESKD worldwide
Hypertensive nephrosclerosis — #2
Glomerulonephritis (IgA nephropathy, FSGS, membranous, lupus nephritis)
Polycystic kidney disease (ADPKD)
Chronic obstruction / reflux nephropathy

Clinical Progression Notes

Patients are typically asymptomatic until late G4–G5
Complications (anemia, MBD, secondary hyperparathyroidism, hypertension) often begin at G3
Dialysis planning should start at G4; initiation based solely on a target GFR has not shown mortality benefit — symptoms and metabolic/nutritional status guide timing
Half of all Americans are expected to develop CKD during their lifetime

— Comprehensive Clinical Nephrology 7th Ed., Washington Manual of Medical Therapeutics, Brenner & Rector's The Kidney

High-yield exam summary:

CKD = ↓eGFR (<60) or kidney damage markers, for >3 months. Stage with G (1–5) + A (1–3). Cause is the "C." Most common cause = diabetes. Complications emerge at G3; dialysis planned at G4.

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