Definition of Chronic Kidney Disease (CKD)
CKD is defined as abnormalities of kidney structure or function present for ≥3 months with implications for health, diagnosed by either: (1) eGFR <60 mL/min/1.73 m², (2) albuminuria (ACR ≥30 mg/g), or (3) other markers of kidney damage (hematuria, structural abnormalities on imaging, or histological abnormalities). 1, 2, 1
Diagnostic Criteria
CKD requires at least one of the following abnormalities persisting for ≥3 months 2, 3:
Markers of Kidney Damage:
- Albuminuria: ACR ≥30 mg/g (≥3 mg/mmol) 2
- Urine sediment abnormalities
- Electrolyte/tubular disorders
- Structural abnormalities on imaging
- Histological abnormalities
- History of kidney transplantation
Decreased Kidney Function:
Screening Requirements
Two simple tests diagnose CKD in the majority of patients 4:
- Spot urine albumin-to-creatinine ratio (ACR)
- Serum creatinine to estimate GFR (using CKD-EPI equation without race) 1
The 3-month persistence requirement is critical to distinguish CKD from acute kidney injury (AKI) or acute kidney disease (AKD), which last <3 months 2, 3, 5. Measurements can vary within individuals, so confirmation of abnormalities is mandatory before diagnosing CKD 1.
Classification System
CKD is classified using the CGA system 2, 3:
- C = Cause (diabetes, hypertension, glomerulonephritis, etc.)
- G = GFR category (G1-G5)
- A = Albuminuria category (A1-A3)
GFR Categories:
- G1: ≥90 mL/min/1.73 m² (requires kidney damage markers for CKD diagnosis)
- G2: 60-89 mL/min/1.73 m² (requires kidney damage markers for CKD diagnosis)
- G3a: 45-59 mL/min/1.73 m²
- G3b: 30-44 mL/min/1.73 m²
- G4: 15-29 mL/min/1.73 m²
- G5: <15 mL/min/1.73 m² or dialysis 2, 3
Albuminuria Categories:
- A1: <30 mg/g (normal to mildly increased)
- A2: 30-300 mg/g (moderately increased)
- A3: >300 mg/g (severely increased) 2
Critical Distinction for Stages 1 and 2
Important caveat: Patients with eGFR ≥60 mL/min/1.73 m² (stages G1 and G2) do not have CKD unless markers of kidney damage are present 3. An isolated eGFR of 70 mL/min/1.73 m² without albuminuria or other damage markers is not CKD.
Key Phrase: "With Implications for Health"
The definition emphasizes "with implications for health" 5. This means not every kidney abnormality constitutes CKD—for example, a simple renal cyst without functional impact would not qualify 5. The diagnosis requires that the abnormality carries clinical significance for patient outcomes, including risk of progression, cardiovascular disease, or complications.
Practical Application
Do not rely on serum creatinine alone—it grossly overestimates kidney function 6. The MDRD Study equation (now superseded by CKD-EPI 2021) shows tighter correlation with measured GFR than 24-hour creatinine clearance 6. For patients with extremes of muscle mass, consider cystatin C-based equations for more accurate GFR estimation 1.
Screening frequency: Annual screening is recommended for high-risk populations (diabetes, hypertension, cardiovascular disease, family history of kidney disease) 1, 7.