Acute kidney injury

Background

  • Majority of cases of community-acquired ARF is secondary to volume depletion although consideration of pre-renal, post renal, and renal causes are important.

AKI Staging

AKI Stage[1] KDIGO UOP
1 Cr 1.5-1.9x baseline over 7d or ≥0.3 mg/dL increase over 48hrs <0.5 mL/kg/hr for 6-12hrs
2 Cr 2.0-2.9x baseline <0.5 mL/kg/hr for >12hrs
3 Cr ≥3.0x baseline or ≥4.0 mg/dL increase or initiation of RRT <0.3 mL/kg/hr for >24hrs or anuria for >12hrs

Chronic Kidney Disease Stages

  • Useful if patient's baseline creatinine is unknown
    • Stage 1: Kidney damage (e.g. proteinuria) and normal GFR; GFR >90
    • Stage 2: Kidney damage (e.g. proteinuria) and mild decrease in GFR; GFR 60-89
    • Stage 3: Moderate decrease in GFR; GFR >30-59
    • Stage 4: Severe decrease in GFR; GFR 15-29
    • Stage 5: Kidney failure (dialysis or kidney transplant needed); GFR <15

Risk Factors

Clinical Features

Etiologies

Prerenal

Intrinsic

Postrenal

Evaluation

Work-up

  • Urine
  • Prostate exam
  • Urinalysis, urine sodium, urine creatinine, urine urea
  • ECG (hyperkalemia)
  • Chronic renal failure features
    • Anemia, thrombocytopenia
    • Iron studies with low Fe, low TIBC, low iron saturation, normal ferritin
    • Secondary rise in PTH, high phos, low calcium

Imaging

  • CXR
    • Evidence of volume overload, pneumonia
  • US: renal/bladder
    • Test of choice in setting of acute renal failure
    • Bladder size (post-void)
    • Hydronephrosis
    • IVC collapsibility (prerenal)
  • CT
    • Useful to determine cause of post renal failure (identification of abdominal masses etc.)
    • Should generally not be used with IV contrast due to potential risk for CIN
    • Indicated if hydronephrosis found on ultrasound in order to define the location of obstruction

Diagnosis

Finding Prerenal Intrinsic Postrenal
BUN/creatinine ratio >20 < 12 12-20 (normal range)
FeNa^
  • <1%
  • < 2% for neonates
  • >1%
  • > 2.5% for neonates
  • >1%
Urine osm >500 <350
Urine sodium < 20 mEq/L > 40 mEq/L
Specific gravity > 1.020 < 1.020
Fractional excretion of urea < 35% > 50%
Microscopic analysis
  • Hyaline casts
  • Acute glomerulonephritis: RBCs, casts
  • Acute tubular necrosis: protein, tubular epithelial cells
  • Interstitial nephritis: eosinophils

^ (urine sodium/plasma sodium) / (urine creatinine / serum creatinine)

Management

Treat underlying cause

  • Prerenal: IVF (or pRBCs if bleeding)
  • Intrinsic: Depends on cause
  • Obstruction:
    • Note: Postobstructive diuresis can result in significant volume loss and death
      • Typically occurs when obstruction has been prolonged / has resulted in renal failure
      • Admit patients with persistent diuresis of >250 mL/h for >2hr
    • Foley Catheter, consider Coude Catheter
    • Suprapubic (if Coude fails)

Dialysis

Phlebotomy to Treat Pulmonary Edema

  • Possible last ditch effort to tide patient over to formal dialysis if hours away
  • If traditional pulmonary edema treatments are not working for SCAPE patient, one may attempt to remove 200-300 cc of blood as a bridge to dialysis[2]
  • If Hb too low, may consider temporary venous tourniquets on each leg proximally q30 min to reduce preload volume to the heart, alternating legs

Disposition

  • Admit

See Also

Calculators

Fractional Excretion of Sodium (FENa)

FENa — Fractional Excretion of Sodium
Parameter Value
Serum Sodium (mEq/L)
Serum Creatinine (mg/dL)
Urine Sodium (mEq/L)
Urine Creatinine (mg/dL)
FENa (%)  %
Interpretation (in setting of oliguria/AKI)
<1% Pre-renal azotemia — Kidneys are sodium-avid (hypoperfusion, hypovolemia, heart failure, cirrhosis). Consider volume resuscitation.
>2% Intrinsic renal disease — ATN, AIN, or glomerulonephritis. Kidneys unable to concentrate urine.
1–2% Indeterminate — May be seen in early ATN or with mixed etiologies. Clinical correlation required.
Important Caveats
  • FENa is unreliable on diuretics — use FEUrea instead
  • Low FENa (<1%) can be seen in contrast nephropathy, rhabdomyolysis, early obstruction
  • Not validated in CKD patients
References
  • Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236:579-581. PMID 947239.
  • Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984;77:699-702. PMID 6486145.


Creatinine Clearance (Cockcroft-Gault)

Creatinine Clearance (Cockcroft-Gault)
Parameter Value
Age (years)
Sex 1 Male   Female
Weight (kg)
Serum Creatinine (mg/dL)
CrCl (mL/min) mL/min
Interpretation
>90 Normal renal function.
60–89 Mildly decreased (CKD Stage 2).
30–59 Moderately decreased (CKD Stage 3). Adjust renally-dosed medications.
15–29 Severely decreased (CKD Stage 4).
<15 Kidney failure (CKD Stage 5). Consider dialysis.
References
  • Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16(1):31-41. PMID 1244564.
  • Formula: CrCl = [(140 − age) × weight (kg)] / [72 × serum Cr (mg/dL)] × 0.85 if female.
  • Note: Use IBW or adjusted BW in obese patients. Not validated in AKI or rapidly changing creatinine.


Schwartz Equation (Pediatric GFR)

Schwartz Equation — Pediatric eGFR
Parameter Value
Height (cm)
Serum Creatinine (mg/dL)
Estimated GFR mL/min/1.73m²
References
  • Schwartz GJ, Muñoz A, Schneider MF, et al. New equations to estimate GFR in children with CKD. J Am Soc Nephrol. 2009;20(3):629-637. PMID 19158356.
  • Bedside Schwartz (2009): eGFR = 0.413 × height (cm) / serum creatinine (mg/dL). Valid for ages 1-16 years.

External Links

References

  1. ↑ Moore PK, et. al. Management of acute kidney injury: core curriculum 2018. Am J Kidney Dis. 2018; 72: 136-148.
  2. ↑ Eiser AR et al. Phlebotomy for pulmonary edema in dialysis patients. Clin Nephrol. 1997 Jan;47(1):47-9.
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