Acute Kidney Injury (AKI)

Definition

• A decrease in glomerular filtration rate (GFR) that leads to an accumulation of waste products in the blood

  • Determined on the basis of increase in serum creatinine (Cr) and/or change in urine output (UOP)

• KDIGO stages of AKI:

  • Stage 1: Cr 1.5-1.9x baseline or increased by ≥0.3 mg/dL, or UOP <0.5 mL/kg/h for 6-12 hours

  • Stage 2: Cr 2.0-2.9x baseline, or UOP <0.5 mL/kg/h for ≥12 hours

  • Stage 3: Cr 3.0x baseline or eGFR <35 mL/min/1.73m² (for children) or Cr ≥ 4.0 mg/dL (for adults), or UOP ≤0.3 mL/kg/h for ≥24 hours or anuric for ≥12 hours

Causes

• Decreased effective kidney blood flow (prerenal AKI)

  • A result of decreased effective circulating blood volume, which may be systemic or at the level of the kidney

    • E.g., dehydration (e.g., gastroenteritis), hemorrhage, sepsis, nephrotic syndrome, liver disease, heart failure, renal artery stenosis/occlusion, vasoconstriction (e.g., NSAIDs, CNIs, vasopressors)

• Intrinsic kidney damage (renal/intrinsic AKI)

  • Glomerular injury (e.g., acute glomerulonephritis [GN])

  • Tubular injury (e.g., acute tubular necrosis [ATN] from medications/toxins)

  • Interstitial injury (e.g., acute interstitial nephritis [AIN], pyelonephritis)

  • Vascular injury (e.g., hemolytic uremic syndrome [HUS])

• Obstruction of urine flow (postrenal AKI)

  • CAKUT (e.g., posterior urethral valves, bilateral upper/lower urinary tract obstruction)

  • Obstructed Foley catheter

Workup:

• Note that these are only general rules.

• UA w/ microscopic

  • Prerenal: unremarkable. Spec grav >1.020

  • Renal/Intrinsic:

    • Granular/epithelial casts: ATN

    • RBC casts: acute GN

• FeNa, UNa

  • Prerenal: <1% (↑ Na reabsorption in response to ↓ kidney perfusion), UNa <10 mEq/L

  • Renal/Intrinsic: FeNa >2%, UNa >40 mEq/L

  • [UpToDate: Fractional excretion of sodium, urea, and other molecules in acute kidney injury]

• Urine to plasma osmolality

  • Prerenal: >1.5 (↑ water reabsorption in response to ↓ kidney perfusion)

  • Renal/Intrinsic: <1.5

• Kidney function panel

  • BUN/Cr ratio:

    • Prerenal: >20:1

      • Note: not specific. Can be elevated in a wide variety of situations (e.g., high dietary protein intake), including in normal children

    • Renal/Intrinsic: 10:1-15:1