How is acute kidney injury diagnosed?

How to Diagnose Acute Kidney Injury

Diagnose AKI when serum creatinine increases by ≥0.3 mg/dL within 48 hours OR increases to ≥1.5 times baseline within 7 days OR urine output falls below 0.5 mL/kg/h for more than 6 consecutive hours. 1, 2

Diagnostic Criteria (KDIGO)

The diagnosis requires meeting any one of these three criteria:

• Creatinine increase ≥0.3 mg/dL within 48 hours - This absolute change criterion is critical because even small creatinine rises independently increase hospital mortality approximately fourfold 1, 2
• Creatinine increase ≥50% (1.5-fold) from baseline within 7 days - This relative change captures AKI across different baseline kidney function levels 1, 2
• Urine output <0.5 mL/kg/h for >6 consecutive hours - However, this criterion has important limitations discussed below 1, 2

Establishing Baseline Creatinine

Use the most recent creatinine value from the previous 3 months when available - select the value closest to hospital admission time. 1, 3 If no prior value exists, use the admission creatinine as baseline. 1, 3

Do not back-calculate baseline using MDRD formulas in cirrhotic patients, as this approach was specifically excluded from consensus recommendations. 1

Staging AKI Severity

Once diagnosed, stage the severity to guide prognosis and management:

• Stage 1: Creatinine 1.5-1.9× baseline OR increase ≥0.3 mg/dL OR urine output <0.5 mL/kg/h for 6-12 hours 1, 2
• Stage 2: Creatinine 2.0-2.9× baseline OR urine output <0.5 mL/kg/h for ≥12 hours 1, 2
• Stage 3: Creatinine ≥3.0× baseline OR creatinine ≥4.0 mg/dL with acute increase ≥0.3 mg/dL OR initiation of renal replacement therapy OR urine output <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours 1, 2

Staging correlates strongly with mortality - progression through higher stages dramatically increases death risk. 2, 3

Essential Diagnostic Workup

After diagnosing AKI, perform this systematic evaluation:

Laboratory Assessment

• Serum creatinine and BUN - BUN:creatinine ratio >20:1 suggests prerenal etiology 4
• Complete blood count with differential - identifies infection, hemolysis, or thrombotic microangiopathy 2
• Urinalysis with microscopy - Look for:
  • RBC casts → glomerulonephritis
  • WBC casts → interstitial nephritis
  • Muddy brown casts → acute tubular necrosis 3
• Urine chemistry - Calculate fractional excretion of sodium (FENa):
  • FENa <1% suggests prerenal AKI
  • FENa >2% suggests acute tubular necrosis 4
• Serum electrolytes - assess for hyperkalemia, acidosis, and other metabolic complications 2

Imaging

• Renal ultrasound - Rule out obstruction (especially in older men) and assess kidney size 2, 4

Critical Caveats for Special Populations

Patients with Cirrhosis and Ascites

Rely exclusively on serum creatinine changes, NOT urine output criteria. 1, 3 These patients are frequently oliguric with avid sodium retention despite maintaining relatively normal GFR, and diuretic therapy further confounds interpretation. 1

In cirrhotic patients, a creatinine threshold ≥1.5 mg/dL predicts AKI progression and worse prognosis. 2

Factors That Affect Creatinine Accuracy

• Muscle wasting - decreases creatinine production, underestimating AKI severity 2
• Hyperbilirubinemia - causes inaccurate creatinine measurement by colorimetric methods 2, 3
• Massive fluid resuscitation - dilutes serum creatinine, potentially masking significant GFR reduction 2
• Volume expansion from ascites - lowers creatinine concentration independent of kidney function 1

Common Pitfalls to Avoid

Do not wait for creatinine to reach 1.5 mg/dL before diagnosing AKI - this outdated threshold often indicates GFR has already fallen to ~30 mL/min. 2 The 0.3 mg/dL increase criterion exists precisely to catch AKI earlier.

Do not rely on urine output alone in patients receiving diuretics - diuretics artificially increase urine output despite worsening kidney function. 2

Monitor creatinine at 48-hour intervals to detect the 0.3 mg/dL threshold in real-time, enabling early intervention. 2

When to Refer to Nephrology

Emergent nephrology consultation is warranted for:

• Stage 2 or 3 AKI
• Stage 1 AKI with concomitant decompensated conditions
• Refractory hyperkalemia, severe acidosis, or volume overload requiring dialysis consideration 3