What are the criteria and initial workup for Acute Kidney Injury (AKI)?

Criteria and Initial Workup for Acute Kidney Injury (AKI)

Definition and Diagnostic Criteria for AKI

The diagnosis of AKI is established by an increase in serum creatinine ≥0.3 mg/dL within 48 hours, or an increase ≥50% from baseline within 7 days, or a decrease in urine output to <0.5 mL/kg/h for 6 hours or more. 1, 2

AKI is staged according to severity:

• Stage 1: Increase in serum creatinine ≥0.3 mg/dL within 48 hours or 50-99% increase from baseline within 7 days; or urine output <0.5 mL/kg/h for >6 hours 2
• Stage 2: 100-199% increase in serum creatinine from baseline within 7 days; or urine output <0.5 mL/kg/h for >12 hours 2
• Stage 3: ≥200% increase in serum creatinine from baseline within 7 days, or serum creatinine ≥4.0 mg/dL with an acute increase of at least 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 2, 1

Initial Workup for AKI

Laboratory Evaluation

• Serum creatinine: Serial measurements to track progression and stage AKI 1
• Blood urea nitrogen (BUN): Helps distinguish between prerenal, intrinsic, and postrenal causes 3
• Complete blood count: To identify anemia, thrombocytopenia, or leukocytosis suggesting systemic illness 3
• Urinalysis with microscopy: Essential for narrowing differential diagnosis 4
  • RBC casts suggest glomerulonephritis
  • WBC casts suggest interstitial nephritis or pyelonephritis
  • Muddy brown casts suggest acute tubular necrosis
• Urine chemistry: Calculate fractional excretion of sodium (FENa) to differentiate prerenal (FENa <1%) from intrinsic renal causes (FENa >2%) 3
• Electrolytes: To identify and manage complications like hyperkalemia 5

Imaging Studies

• Renal ultrasound: Recommended for most patients with AKI to rule out obstruction, especially in older men 3, 1

  • Evaluates kidney size and echogenicity
  • Can differentiate AKI from chronic kidney disease (CKD) by determining renal size and volume 1
  • Small kidneys (<9 cm) suggest CKD rather than AKI 1

• Unenhanced CT abdomen/pelvis: Consider if ultrasound is inconclusive or not feasible due to body habitus 1

  • More sensitive than ultrasound for urinary tract calculi and retroperitoneal pathology 1
  • Useful for characterizing ultrasound-detected hydronephrosis by determining level and cause of obstruction 1

Risk Stratification

• Stratify patients for risk of AKI according to their susceptibilities and exposures 1
• Common risk factors include:
  • Advanced age
  • Sepsis
  • Hypovolemia/shock
  • Cardiac surgery
  • Contrast agent exposure
  • Diabetes mellitus
  • Pre-existing CKD
  • Heart failure
  • Liver failure 3

Determining the Cause of AKI

The cause of AKI should be determined whenever possible 1. Categorize into:

1. Prerenal causes: Due to decreased renal perfusion

   • Hypovolemia, heart failure, sepsis, hepatorenal syndrome 3

2. Intrinsic renal causes: Direct damage to kidney structures

   • Acute tubular necrosis, acute interstitial nephritis, glomerulonephritis, vasculitis 3

3. Postrenal causes: Due to urinary tract obstruction

   • Prostatic hypertrophy, nephrolithiasis, malignancy 3

Special Considerations

AKI in Cirrhosis

• Standard AKI criteria apply, but urine output criteria may be problematic as patients with cirrhosis and ascites are frequently oliguric with avid sodium retention 1
• Serum creatinine may underestimate kidney dysfunction in cirrhosis due to:
  • Decreased creatinine formation from muscle wasting
  • Increased tubular secretion of creatinine
  • Dilution of serum creatinine by ascites 1

Persistent vs. Transient AKI

• Persistent AKI: Continuation of AKI beyond 48 hours from onset 1
• Rapid reversal of AKI: Complete reversal within 48 hours of onset 1
• Persistent AKI may progress to Acute Kidney Disease (AKD), defined as kidney dysfunction persisting ≥7 days after an AKI-initiating event 1

Initial Management Approach

• Reassess the underlying etiology of AKI when it persists 1
• Monitor hemodynamic and volume status, adequacy of kidney perfusion 1
• Identify complications such as fluid overload, acidosis, and hyperkalemia 1
• Consider nephrology consultation if the etiology is unclear or subspecialist care is needed 1, 4
• Emergent referral to nephrology is recommended for:
  • Stage 2 or 3 AKI
  • Stage 1 AKI with concomitant decompensated condition
  • Unclear etiology of AKI 4

Remember that even one episode of AKI increases the risk of cardiovascular disease, chronic kidney disease, and death, making early determination of etiology, management, and long-term follow-up essential 6.