Clinical meaning
The clinician diagnoses acute kidney injury using the KDIGO criteria: Stage 1 (serum creatinine increase of 26.5 micromol/L or greater within 48 hours, OR 1.5-1.9 times baseline within 7 days, OR urine output less than 0.5 mL/kg/hr for 6-12 hours), Stage 2 (creatinine 2.0-2.9 times baseline, OR urine output less than 0.5 mL/kg/hr for 12 or more hours), Stage 3 (creatinine 3.0 or more times baseline, OR increase to 353.6 micromol/L or greater, OR initiation of renal replacement therapy, OR urine output less than 0.3 mL/kg/hr for 24 or more hours, OR anuria for 12 or more hours). The clinician categorizes AKI etiology as prerenal (60-70% of cases -- decreased renal perfusion from hypovolemia, heart failure, hepatorenal syndrome, sepsis; BUN/Cr ratio greater than 20:1, FENa less than 1%, urine specific gravity greater than 1.020), intrinsic (25-30% -- acute tubular necrosis from ischemia or nephrotoxins, acute interstitial nephritis, glomerulonephritis; FENa greater than 2% in ATN, muddy brown granular casts), or postrenal (5-10% -- urinary tract obstruction; hydronephrosis on renal ultrasound). The clinician performs comprehensive evaluation: medication review (NSAIDs, aminoglycosides, contrast, ACEIs/ARBs, lithium), urinalysis with microscopy (RBC casts suggest glomerulonephritis, WBC casts suggest interstitial nephritis, muddy brown casts suggest ATN), renal ultrasound (size, echogenicity, hydronephrosis), and appropriate serologies when glomerulonephritis is suspected (ANA, ANCA, anti-GBM, complement levels, hepatitis B/C, HIV). Management: volume optimization, nephrotoxin avoidance, appropriate medication dose adjustment, and nephrology referral for severe or complex cases.