What are the lab findings of Acute Kidney Injury (AKI)?

Laboratory Findings of Acute Kidney Injury (AKI)

The primary laboratory findings of AKI include an increase in serum creatinine of ≥0.3 mg/dL within 48 hours, or an increase of ≥50% from baseline within 7 days, or a reduction in urine output to <0.5 mL/kg/h for 6 hours or more. 1, 2

Diagnostic Criteria

• AKI is defined by the Kidney Disease Improving Global Outcomes (KDIGO) criteria as an abrupt decrease in kidney function occurring over 7 days or less 1
• The diagnosis requires at least one of the following criteria:
  • Increase in serum creatinine by ≥0.3 mg/dL (26.5 μmol/L) within 48 hours 3, 1
  • Increase in serum creatinine to ≥1.5 times baseline within the previous 7 days 3, 2
  • Urine volume <0.5 mL/kg/h for 6 consecutive hours 3, 2

AKI Staging Based on Laboratory Findings

• Stage 1:

  • Serum creatinine increase ≥0.3 mg/dL within 48 hours or 50-99% from baseline within 7 days 2
  • Urine output <0.5 mL/kg/h for >6 hours 2

• Stage 2:

  • Serum creatinine increase of 100-199% from baseline within 7 days 2
  • Urine output <0.5 mL/kg/h for >12 hours 2

• Stage 3:

  • Serum creatinine increase ≥200% from baseline within 7 days 2
  • Serum creatinine ≥4.0 mg/dL with an acute increase of at least 0.3 mg/dL 2
  • Initiation of renal replacement therapy 2
  • Urine output <0.3 mL/kg/h for 24 hours or anuria for 12 hours 2

Urinalysis Findings

• Urinalysis can help differentiate the etiology of AKI 3:
  • Normal or "bland" sediment may suggest prerenal causes 4
  • Presence of hematuria (>50 RBCs per high-power field) may indicate glomerular disease 3
  • Presence of proteinuria (>500 mg/day) may suggest glomerular disease 3
  • Presence of cellular casts, particularly renal tubular epithelial cell casts, suggests acute tubular necrosis 3

Urine Biochemistry

• Fractional excretion of sodium (FENa):

  • FENa <1% suggests prerenal causes (increased sodium reabsorption) 4
  • FENa >1% suggests intrinsic renal causes (tubular damage) 4
  • Note: The value of urine biochemistry has been questioned, especially in sepsis 3, 4

• Urine sodium concentration:

  • <10 mEq/L typically seen in prerenal AKI 4

  • 20 mEq/L often seen in intrinsic renal causes 4

• Fractional excretion of urea (FEUrea):

  • <28.16% may be indicative of hepatorenal syndrome with sensitivity of 75% and specificity of 83% 4
  • May be more reliable than FENa in certain settings such as diuretic use 4

Factors Affecting Laboratory Interpretation

• Serum creatinine has important limitations as a marker of AKI 3:

  • Affected by muscle mass, age, sex, and nutritional status 3
  • May underestimate severity of renal function reduction in patients with cirrhosis due to impaired hepatic production of creatine 3
  • Hyperbilirubinemia can result in inaccurate measurement by colorimetric methods 3
  • Changes in creatinine lag behind actual kidney injury 3

• Small increases in serum creatinine (≥0.3 mg/dL) are independently associated with approximately fourfold increase in hospital mortality 1, 2

Novel Biomarkers

• While not yet part of routine diagnostic criteria, several biomarkers are being studied for earlier detection of AKI 3:
  • Neutrophil gelatinase-associated lipocalin (NGAL) in serum or urine 3, 5
  • Kidney injury molecule-1 (KIM-1) 3, 5
  • Tissue inhibitor metalloproteinase-2 (TIMP-2) and insulin-like growth factor-binding protein 7 (IGFBP7) 3
  • Cystatin C may predict need for chronic dialysis after AKI 3, 5
  • Interleukin-18, liver-type fatty acid-binding protein, and calprotectin 5

Common Pitfalls in Laboratory Interpretation

• Relying solely on serum creatinine without considering urine output may miss cases of AKI 1
• Failure to establish an accurate baseline creatinine can lead to misclassification 1
• Not accounting for biological and analytical variability in creatinine measurements 3
• Small absolute increases in creatinine in patients with pre-existing chronic kidney disease represent smaller percentage increases, potentially leading to underdiagnosis 3
• Not recognizing that AKI can occur in both hospital and community settings 1

Imaging Findings

• Renal ultrasound is recommended to evaluate kidney size and rule out obstruction 3, 4
• Normal kidney size suggests AKI rather than chronic kidney disease 4
• New imaging techniques such as contrast-enhanced ultrasound, doppler ultrasound, and functional MRI are being studied but their role in changing outcomes is yet to be determined 3