What are the criteria and initial work‑up for diagnosing acute kidney injury (AKI)?

Diagnosis of Acute Kidney Injury

Diagnose AKI when any one of three KDIGO criteria is met: serum creatinine rises ≥0.3 mg/dL within 48 hours, OR creatinine increases to ≥1.5× baseline within 7 days, OR urine output falls below 0.5 mL/kg/h for ≥6 consecutive hours. 1, 2

Diagnostic Criteria

The KDIGO definition requires only one of three independent criteria—you do not need all three present simultaneously: 1, 2

• Absolute creatinine rise: ≥0.3 mg/dL increase within any 48-hour window 1, 2
• Relative creatinine rise: ≥1.5× baseline (or ≥50% increase) within the preceding 7 days 1, 2
• Oliguria: Urine output <0.5 mL/kg/h sustained for ≥6 consecutive hours 1, 2

Even the modest 0.3 mg/dL threshold matters clinically—it independently predicts a four-fold increase in hospital mortality. 1, 2

Establishing Baseline Creatinine

Use the most recent serum creatinine measured within the prior 3 months, selecting the value closest to hospital admission. 1, 2 This approach outperforms mathematical imputation methods. 2

• If no prior measurement exists, accept the admission creatinine as baseline 1, 2
• Never back-calculate baseline creatinine using MDRD equations in patients with cirrhosis—this method is explicitly excluded by consensus 1, 2
• In non-cirrhotic populations, back-calculation assuming GFR of 75 mL/min/1.73 m² may overestimate AKI incidence where chronic kidney disease prevalence is high 2

AKI Staging

Stage AKI retrospectively by the most severe criterion met during the episode—either creatinine or urine output. 1, 2 Higher stages correlate strongly with increased mortality: 1

Stage	Creatinine Criterion	Urine Output Criterion
Stage 1	1.5–1.9× baseline OR rise ≥0.3 mg/dL	<0.5 mL/kg/h for 6–12 hours
Stage 2	2.0–2.9× baseline	<0.5 mL/kg/h for ≥12 hours
Stage 3	≥3.0× baseline OR ≥4.0 mg/dL (with acute rise ≥0.3 mg/dL) OR initiation of dialysis	<0.3 mL/kg/h for ≥24 hours OR anuria ≥12 hours

1, 2

Initial Laboratory Work-Up

Order the following core panel to assess for AKI and determine etiology: 3

• Serum creatinine and blood urea nitrogen (BUN): Primary diagnostic markers; BUN-to-creatinine ratio helps differentiate prerenal (ratio >20:1) from intrinsic causes 3, 4
• Complete blood count with differential: Identifies infection, hemolysis, or thrombotic microangiopathy 3
• Serum electrolytes (sodium, potassium, chloride, bicarbonate): Assesses metabolic complications and guides management 3
• Urinalysis with microscopy: Hematuria (>50 RBCs/hpf) or proteinuria (>500 mg/day) suggests glomerular disease; renal tubular epithelial cell casts indicate acute tubular necrosis 3
• Urine chemistry: Calculate fractional excretion of sodium (FENa <1% suggests prerenal; >2% suggests acute tubular necrosis) or fractional excretion of urea (FEUrea) when diuretics confound FENa 3, 4

Renal ultrasound is appropriate when obstruction is suspected or to assess kidney size—normal kidney size suggests AKI rather than chronic kidney disease. 3

Special Considerations for Cirrhosis and Ascites

In patients with decompensated cirrhosis, rely exclusively on serum creatinine changes and ignore urine output criteria. 1, 2 These patients are frequently oliguric with avid sodium retention yet maintain relatively normal GFR, and diuretic therapy further confounds urine output interpretation. 1, 2

• Baseline creatinine underestimates true GFR in cirrhosis due to reduced muscle mass, making the absolute 0.3 mg/dL rise especially important in this population 2
• The traditional fixed threshold of ≥1.5 mg/dL is problematic—it often indicates GFR has already fallen to approximately 30 mL/min 1, 2
• Apply the International Club of Ascites (ICA) criteria for this population, which exclude urine output 2, 5

Common Pitfalls and How to Avoid Them

Do not dismiss modest absolute creatinine rises in patients with chronic kidney disease—the KDIGO absolute criterion captures clinically relevant AKI regardless of baseline function. 2 In advanced CKD, a 90% reduction in creatinine clearance yields only a 47% creatinine rise versus 246% in normal kidneys. 2

Massive fluid resuscitation can dilute serum creatinine, potentially masking significant GFR reduction. 2 When cumulative fluid gain exceeds 5–10% of baseline body weight, adjust creatinine values for volume expansion to obtain a more accurate estimate of renal function. 3, 2

Do not rely on urine output alone in patients receiving diuretics—oliguria may not reflect true GFR loss in these settings. 1, 2

Check for laboratory interferences: 3

• Hyperbilirubinemia causes false elevation with Jaffe assays and false reduction with enzymatic assays 3
• Trimethoprim and cimetidine inhibit tubular secretion of creatinine, producing spuriously high values unrelated to true GFR changes 3

Monitoring and Follow-Up

After diagnosing AKI, perform serial monitoring of serum creatinine and urine output to determine stage and track progression. 3

Re-evaluate all patients at 3 months post-AKI to assess for resolution, new-onset chronic kidney disease, or progression of pre-existing kidney disease. 3 Even patients without prior CKD who experience AKI have elevated risk of subsequent kidney dysfunction and cardiovascular disease. 3, 6

Refer patients with Stage 2 or Stage 3 AKI for prompt nephrology consultation. 3