CKD classification and staging

CKD Definition - The Basics

• Chronic Kidney Disease (CKD) is defined as abnormalities of kidney structure or function, present for >3 months, with health implications.
• Diagnosis requires at least one of the following criteria:
  • Markers of kidney damage (e.g., albuminuria, hematuria, pathologic abnormalities on biopsy).
  • Glomerular Filtration Rate (GFR) <60 mL/min/1.73 m².
• Albuminuria is a key marker: Albumin-to-Creatinine Ratio (ACR) >30 mg/g.

⭐ Diabetes and hypertension are the two most common causes of CKD in the US.

CKD Staging by GFR - Grading the Filter

• GFR Stage 1: ≥90 mL/min/1.73m² - Normal GFR, but with kidney damage (e.g., proteinuria, abnormal imaging).
• GFR Stage 2: 60-89 mL/min/1.73m² - Mildly ↓ GFR.

  ⭐ Uremia symptoms (e.g., nausea, pruritus, pericarditis) typically manifest in Stage 4-5, but subtle cognitive changes can begin earlier.

• GFR Stage 3: 30-59 mL/min/1.73m² - Moderately ↓ GFR.
  • 3a: 45-59
  • 3b: 30-44
• GFR Stage 4: 15-29 mL/min/1.73m² - Severely ↓ GFR; prepare for renal replacement therapy (RRT).
• GFR Stage 5: <15 mL/min/1.73m² - Kidney failure / End-Stage Renal Disease (ESRD); RRT indicated.

Albuminuria Categories - Leaky Pipes Alert

Measures glomerular damage via the urine Albumin-to-Creatinine Ratio (ACR). It is a continuous risk multiplier; higher levels signal ↑ risk for CKD progression and cardiovascular events.

• A1 (Normal to Mildly Increased): ACR < 30 $mg/g$
• A2 (Moderately Increased): ACR 30-300 $mg/g$
  • Historically termed "microalbuminuria."
  • Crucial early indicator of diabetic nephropathy and systemic endothelial dysfunction.
• A3 (Severely Increased): ACR > 300 $mg/g$
  • Historically "macroalbuminuria" or overt proteinuria.
  • Strongly associated with rapid GFR decline.

⭐ The risk for adverse outcomes begins even at low levels of albuminuria (e.g., 10-29 mg/g), well below the traditional 30 $mg/g$ cutoff for A2.

Prognosis by GFR & Albuminuria - The KDIGO Heatmap

Prognosis in CKD is determined by the combination of GFR and albuminuria categories, visualized as a color-coded risk grid (Green → Yellow → Orange → Red). The risk of adverse outcomes-including CKD progression, cardiovascular events, and mortality-escalates with lower GFR and higher albuminuria.

• Albuminuria Categories (Albumin-to-Creatinine Ratio, ACR)
  • A1: < 30 mg/g (Normal to mildly increased)
  • A2: 30-300 mg/g (Moderately increased)
  • A3: > 300 mg/g (Severely increased)
• GFR Categories ($mL/min/1.73m^2$)
  • G1-G5 correspond to specific GFR ranges.

⭐ Patients with CKD are more likely to die from cardiovascular disease than to progress to end-stage renal disease (ESRD). The risk is highest in those with severe albuminuria (A3).

High‑Yield Points - ⚡ Biggest Takeaways

• CKD is defined by GFR <60 mL/min/1.73m² or markers of kidney damage for >3 months.
• Staging combines GFR categories (G1-G5) and albuminuria categories (A1-A3) to assess prognosis.
• Persistent albuminuria is a key independent risk factor for CKD progression and cardiovascular events.
• Stage G3 (GFR 30-59) is a critical point where complications like anemia and bone disease accelerate.
• Stage G5 (GFR <15) is End-Stage Renal Disease (ESRD), requiring planning for renal replacement therapy.