Clinical meaning
Acute kidney injury (AKI) and chronic kidney disease (CKD) represent acute and chronic forms of renal dysfunction with distinct diagnostic criteria and management implications. AKI is defined by the KDIGO criteria: Stage 1 = creatinine rise ≥ 0.3 mg/dL within 48 hours OR 1.5-1.9x baseline within 7 days OR urine output < 0.5 mL/kg/hr for 6-12 hours; Stage 2 = creatinine 2.0-2.9x baseline OR UO < 0.5 mL/kg/hr for ≥ 12 hours; Stage 3 = creatinine ≥ 3x baseline OR ≥ 4.0 mg/dL OR initiation of RRT OR UO < 0.3 mL/kg/hr for ≥ 24 hours OR anuria ≥ 12 hours. AKI etiology classification: prerenal (60-70%, from decreased renal perfusion — dehydration, heart failure, shock; BUN:Cr ratio > 20:1, FENa < 1%), intrinsic renal (25-30%, from tubular necrosis, glomerulonephritis, interstitial nephritis; FENa > 2%, muddy brown casts in ATN), and postrenal (5-10%, from obstruction; hydronephrosis on ultrasound). CKD is staged by GFR using the CKD-EPI equation: Stage 1 = GFR ≥ 90 with kidney damage markers; Stage 2 = 60-89; Stage 3a = 45-59; Stage 3b = 30-44; Stage 4 = 15-29; Stage 5 = < 15 (ESRD/dialysis). CKD is further classified by albuminuria category: A1 < 30, A2 30-300, A3 > 300 mg/g.