Definition of Chronic Kidney Disease
Chronic kidney disease is defined as either kidney damage or decreased kidney function (GFR <60 mL/min/1.73 m²) persisting for 3 months or more, regardless of the underlying cause. 1, 2, 3
Core Diagnostic Components
CKD diagnosis relies on three essential elements that must be present: 1
- Structural/Anatomical Component: Markers of kidney damage, most importantly albuminuria 1
- Functional Component: Based on glomerular filtration rate measurement 1
- Temporal Component: Abnormalities must persist for at least 3 months 1, 2
Markers of Kidney Damage
Persistent proteinuria serves as the principal marker of kidney damage, even when GFR remains normal. 1, 2
Specific diagnostic thresholds include: 1, 2
- Albumin-creatinine ratio >30 mg/g in spot urine samples
- Sex-specific cutpoints: >17 mg/g in men and >25 mg/g in women
Additional markers of kidney damage beyond proteinuria: 1
- Abnormalities in urine sediment
- Abnormalities in blood and urine chemistry measurements
- Abnormal findings on imaging studies
GFR Thresholds and Interpretation
GFR <60 mL/min/1.73 m² represents loss of half or more of normal adult kidney function and marks the threshold where complications of CKD increase substantially. 1
Key GFR reference values: 1
- Normal GFR in young adults: approximately 120-130 mL/min/1.73 m²
- GFR declines with age as part of normal aging
- However, decreased GFR in elderly patients remains an independent predictor of adverse outcomes including death and cardiovascular disease 1
The definition of CKD applies uniformly regardless of patient age, despite age-related GFR decline. 1
CKD Staging System
CKD is classified into five stages based on GFR level, independent of the underlying cause: 1, 2
| Stage | GFR (mL/min/1.73 m²) | Description |
|---|---|---|
| 1 | ≥90 | Kidney damage with normal or increased GFR |
| 2 | 60-89 | Kidney damage with mild decrease in GFR |
| 3 | 30-59 | Moderate decrease in GFR |
| 4 | 15-29 | Severe decrease in GFR |
| 5 | <15 | Kidney failure |
Critical staging caveat: Stages 1 and 2 require documented evidence of kidney damage (such as albuminuria) in addition to the GFR level—GFR alone does not define early-stage CKD. 1, 2
Modern Risk Stratification
The KDIGO classification system provides more comprehensive risk stratification by combining: 3
- GFR categories (G1-G5): ranging from normal/high (≥90 mL/min/1.73 m²) to kidney failure (<15 mL/min/1.73 m²)
- Albuminuria categories (A1-A3): from normal (<30 mg/g) to severely increased (≥300 mg/g)
Both urine albumin measurement and GFR assessment should be used together to test people at risk for CKD. 3
Clinical Implications for Outcomes
The definition prioritizes early detection because: 1
- Decreased GFR associates with multiple complications: hypertension, anemia, malnutrition, bone disease, neuropathy, and decreased quality of life
- These complications can be prevented or ameliorated by treatment at earlier stages
- Treatment can slow progression to kidney failure
- Cardiovascular disease events are more common than kidney failure in CKD patients 1
- CKD patients should be considered in the highest risk group for subsequent cardiovascular events 1
Common pitfall: Do not dismiss mildly reduced GFR as "normal aging" without considering it represents true CKD requiring medication dose adjustments and increased cardiovascular risk monitoring. 1