CKD Staging Explained: The Five KDIGO GFR Categories and Three Albuminuria Grades
Chronic kidney disease is staged G1 to G5 by glomerular filtration rate (stage 3 is split into 3a and 3b) and graded A1 to A3 by albuminuria. Only when both axes are combined can risk be properly assessed.
Why Stage at All?
Staging is not about labels. It serves three purposes: estimating prognosis, setting monitoring frequency and guiding treatment. Since 2012 the international body KDIGO has used the "CGA" framework, retained in its 2024 update:
- Cause
- GFR category (G1–G5)
- Albuminuria category (A1–A3)
Axis One: GFR Category
GFR measures how much blood the kidneys filter per minute. Clinically it is estimated (eGFR) from serum creatinine — sometimes with cystatin C — in mL/min/1.73 m².
| Category | eGFR | Description |
|---|---|---|
| G1 | ≥ 90 | Normal or high (CKD only if kidney damage is also present) |
| G2 | 60–89 | Mildly decreased (CKD only if kidney damage is also present) |
| G3a | 45–59 | Mildly to moderately decreased |
| G3b | 30–44 | Moderately to severely decreased |
| G4 | 15–29 | Severely decreased |
| G5 | < 15 | Kidney failure (G5D if on dialysis) |
Important: G1 and G2 alone do not establish CKD. There must also be evidence of kidney damage (albuminuria, hematuria, imaging abnormalities) persisting for more than 3 months.
Why Split Stage 3 into 3a and 3b?
Research shows eGFR 45 is a clear risk threshold: below it, cardiovascular events, progression to kidney failure and mortality all rise sharply. Splitting G3 helps allocate resources and time nephrology referral more precisely.
Axis Two: Albuminuria Category
Albumin is the main protein in blood and normally barely leaks into urine. When glomeruli are damaged, albumin escapes. It is measured as the urine albumin-to-creatinine ratio (UACR) in mg/g.
| Category | UACR (mg/g) | Description |
|---|---|---|
| A1 | < 30 | Normal to mildly increased |
| A2 | 30–300 | Moderately increased (formerly "microalbuminuria") |
| A3 | > 300 | Severely increased (formerly "macroalbuminuria") |
Albuminuria is an independent risk factor: at the same eGFR, an A3 patient progresses faster and carries higher cardiovascular risk than an A1 patient. This is why KDIGO 2024 insists that CKD evaluation requires both a blood test and a urine test.
Combining the Axes: The Risk Heat Map
KDIGO cross-tabulates GFR and albuminuria into a "traffic-light" table (the original uses green–yellow–orange–red):
| A1 (< 30) | A2 (30–300) | A3 (> 300) | |
|---|---|---|---|
| G1 (≥ 90) | Low risk* | Moderate | High |
| G2 (60–89) | Low risk* | Moderate | High |
| G3a (45–59) | Moderate | High | Very high |
| G3b (30–44) | High | Very high | Very high |
| G4 (15–29) | Very high | Very high | Very high |
| G5 (< 15) | Very high | Very high | Very high |
*G1A1 and G2A1 without other markers of kidney damage are not CKD.
Suggested monitoring frequency by risk (KDIGO 2024):
- Low: once a year
- Moderate: once a year
- High: twice a year
- Very high: 3–4 or more times a year
What Typically Happens at Each Stage?
General patterns only — individual variation is wide:
- G1–G2: usually asymptomatic. Focus on controlling the cause (glucose, blood pressure) and avoiding nephrotoxic drugs.
- G3a–G3b: mild anemia, rising parathyroid hormone, elevated phosphate and harder-to-control blood pressure may appear. Nephrology referral is advised; begin dietitian discussions about protein and sodium.
- G4: symptoms become noticeable — fatigue, poor appetite, swelling, nocturia, itching. Kidney replacement therapy education should start: discuss dialysis modalities, transplant evaluation, and plan vascular access early.
- G5: uremic symptoms are evident. The timing of dialysis is decided by symptoms, electrolytes and fluid status — not by the number alone.
Reading the Numbers Correctly
- Trends, not single values: eGFR fluctuates with hydration, muscle mass, medications and lab variation. KDIGO recommends repeat testing at least 3 months apart to confirm.
- An eGFR decline of more than 5 per year is "rapid progression" and warrants aggressive intervention.
- Cystatin C: in people with very low or very high muscle mass (older adults, bodybuilders, amputees), creatinine-based estimates are unreliable; KDIGO 2024 recommends adding cystatin C for accuracy.
Key Takeaways
- CKD staging has two axes — GFR (G1–G5) and albuminuria (A1–A3) — and both are essential
- eGFR below 45 (G3b) marks a clear rise in risk
- More albuminuria means higher risk even at the same eGFR
- Staging determines monitoring frequency and referral timing; preparation for kidney replacement therapy should begin at G4
References
- KDIGO. 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024;105(4S):S117–S314.
- National Kidney Foundation. How to Classify CKD.
- Levey AS, et al. Nomenclature for kidney function and disease: report of a KDIGO Consensus Conference. Kidney Int. 2020.
- Taiwan Society of Nephrology. CKD Prevention Handbook.
This article summarizes publicly available medical information for educational purposes only and is not medical advice. For any questions about your health, illness or physical care, please seek help through professional medical channels and consult your physician.
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