Acute Kidney Injury (AKI)

DEFINITION ₍₁₎

• Increase in creatinine by 1.5 x baseline OR
• Increase in creatinine by 0.3 mg/dL from baseline OR
• Urine output < 0.5 mL/kg/hour for 6+ hours

CATEGORIES

Pre-Renal

• Clinical Clues
  • History/physical exam suggestive of hypovolemia, heart failure, or cirrhosis
  • Urinalysis: Increased specific gravity and hyaline casts
  • FENa (Fractional Excretion of Sodium): <1%
  • FEUrea (Fractional Excretion of Urea) <35%
    • Utilized instead of FENa if patient has been receiving diuretics)
  • BUN/Cr ratio > 20
• Etiologies
  • Cardiorenal Syndrome
  • Hepatorenal Syndrome
  • Hypovolemia (Dehydration, Diarrhea, Diuresis, Vomiting)
  • Arterial insufficiency (Renal Artery Stenosis, Fibromuscular Dysplasia)
• Management
  • Cardiorenal Syndrome: Diuresis (to move leftward on the Frank-Starling curve)
  • Hepatorenal Syndrome: Usually an albumin trial of 1 g/kg (up to 100mg) per day for 2 days is trialed to rule out a volume-responsive kidney injury. Octreotide (subcutaneous), midodrine, norepinephrine, and eventually a liver transplant is usually necessary if albumin fails.
  • Hypovolemia (Dehydration, Diarrhea, Diuresis, Vomiting): IV fluid resuscitation
  • Arterial insufficiency (Renal Artery Stenosis, Fibromuscular Dysplasia): Vascular Surgery intervention may be necessary

Intra-Renal

• Clinical Clues
  • FENa >1% and FEUrea >35%
  • BUN/Creatinine <20
  • “Muddy brown” casts on urine microscopy
    • In addition, myoglobinuria will manifest as positive blood on the urine dipstick, with a low (or zero) count of red blood cells on microscopy
• Etiologies
  • Nephrotic Syndrome (see Nephrotic Syndrome)
    • 3.5+ g/day of proteinuria (or random urine protein:urine creatinine ratio of 3.5+), hypoalbuminemia, edema
  • Nephritic Syndrome (see Nephritic Syndrome)
    • UA with large hematuria, hypertension
  • Acute Interstitial Nephritis
    • Fever, eosinophilia or WBC casts (“sterile pyuria”), rash
  • Acute Tubular Necrosis
    • “Muddy brown casts” on urine microscopy

Post-Renal

• Etiologies
  • BPH, prostate cancer
  • Cervical cancer (less common)
  • Nephrolithiasis (typically would need to be bilateral to cause AKI)
  • Neurogenic Bladder (due to spinal cord injury, diabetes, etc.)
  • Medications
    • Anticholinergics are generally the biggest offender
• Work-Up
  • History: Incontinence, prostate disease, kidney stones, flank pain, trauma
  • Post-void residual for urinary retention
  • Renal ultrasound for hydronephrosis
  • Consider CT scan to rule out renal stones or neoplasm
• Management
  • Foley catheter and/or percutaneous nephrostomy tube drainage may be necessary to decompress hydronephrosis, but definitive treatment is to treat the underlying cause

Initial Workup:

• History and physical (volume status, history of heart failure, cirrhosis, spinal trauma, etc)
• Basic Metabolic Panel
• Urinalysis + microscopy
• Urine creatinine, urine sodium, urine urea nitrogen
• Bladder scan
• Consider retroperitoneal ultrasound to assess for hydronephrosis

References

1. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl 2012; 2:8.