Classification of Chronic Kidney Disease

CKD categories were defined using the Kidney Disease: Improving Global Outcomes (KDIGO) 2012 Clinical Practice Guidelines ¹¹ using eGFR and albuminuria. We used the 2012 CKD-EPI (CKD Epidemiology Collaboration) equation using serum creatinine and cystatin C, age, sex, and race to estimate GFR ¹². Serum creatinine was measured on a Roche Modular P Chemistry Analyzer (Roche Diagnostics) using the creatinase enzymatic method and standardized to isotope-dilution mass spectrometry-traceable reference method. Serum cystatin C was measured using the Gentian immunoassay turbidimetric method (Gentian, Moss, Norway), calibrated and standardized to International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) reference. Albuminuria was assessed as a ratio of urine albumin to urine creatinine (ACR) from spot urine samples expressed as mg of albumin per gram of creatinine. Urine albumin was measured using the nephelometric methods on either the Dade Behring BN100 or the Beckman Image Nephelometer. Urine creatinine was measured using the Jaffe method. Estimated GFR was categorized in these GFR categories: G1, eGFR ≥ 90 mL/min/1.73 m²; G2, eGFR 60–89 mL/min/1.73 m²; G3a, eGFR 45–59 mL/min/1.73 m²; G3b, eGFR 30–44 mL/min/1.73 m²; G4, eGFR 15–29 mL/min/1.73 m²; and G5, eGFR <15 mL/min/1.73 m². Albuminuria was categorized into these stages: A1, ACR <30 mg/g; A2, ACR 30–300 mg/g; and A3, ACR >300 mg/g. Estimated GFR and albuminuria categories were combined to define four CKD risk categories: low risk (G1/G2 and A1); moderately high-risk (G3a and A1; G1/G2 and A2); high risk (G3b and A1; G3a and A2; G1/G2 and A3); or very high-risk (G3a and A3; G3b and A2/A3; G4/G5). For interpretability, we will refer to the risk categories as “no CKD” (for low risk), “moderate CKD” (for moderately high risk), “severe CKD” (for high risk), and “very severe CKD” (for very high risk).