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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 68-year-old man is evaluated on the surgical ward for decreased urine output. He underwent an open cholecystectomy for acute gangrenous cholecystitis two days ago. Over the past 12 hours, his urine output has been 150 mL via the urethra after his indwelling urinary catheter was removed earlier in the day. His preoperative serum creatinine was 1.1 mg/dL, and his current serum creatinine is 2.8 mg/dL. His medical history includes hypertension, benign prostatic hyperplasia, and osteoarthritis. His perioperative medications include lisinopril, ketorolac, and piperacillin-tazobactam. Vital signs are temperature 37.1°C (98.8°F), blood pressure 135/82 mmHg, and heart rate 88/min. Physical examination shows normal skin turgor, a healing right upper quadrant incision, and dullness to percussion in the suprapubic region. Which of the following studies comprise the most appropriate initial diagnostic workup for this patient's acute kidney injury? (Select exactly 3 answers)

  1. Point-of-care bladder ultrasoundAnswer
  2. Urinalysis with urine microscopyAnswer
  3. Urine chemistry with fractional excretion of sodium calculationAnswer
  4. D
    Noncontrast computed tomography scan of the abdomen and pelvis
  5. E
    Percutaneous renal biopsy

Answer

The most appropriate initial diagnostic tests are point-of-care bladder ultrasound, urinalysis with urine microscopy, and urine chemistry with fractional excretion of sodium calculation.
This patient is presenting with severe postoperative acute kidney injury (AKI). The diagnostic approach to undifferentiated AKI requires simultaneous evaluation of prerenal, intrinsic renal, and postrenal etiologies. This patient has risk factors for all three: lisinopril (prerenal), ketorolac and piperacillin-tazobactam (intrinsic renal, such as acute tubular necrosis or acute interstitial nephritis), and a history of benign prostatic hyperplasia with recent indwelling catheter removal (postrenal). The optimal initial, non-invasive diagnostic bundle includes a point-of-care bladder ultrasound (to rapidly diagnose urinary retention, heavily supported by his suprapubic dullness), a urinalysis with microscopy (to identify casts, cells, or proteinuria indicative of intrinsic disease), and urine chemistry to calculate the fractional excretion of sodium (FENa), which distinguishes prerenal azotemia from intrinsic renal failure.

Step-by-Step Solution

1
Identify the clinical problem.
The patient has developed severe, new-onset acute kidney injury (AKI) postoperatively, characterized by oliguria and a sharp rise in serum creatinine.
Recognizing the severity of the AKI dictates the urgency of a comprehensive diagnostic workup.
2
Formulate a differential diagnosis based on risk factors.
The patient has overlapping risk factors for prerenal (lisinopril use), intrarenal (ketorolac and antibiotic exposure), and postrenal (benign prostatic hyperplasia, recent catheter removal, suprapubic dullness) causes of AKI.
AKI can be multifactorial; failing to investigate all three pathophysiologic compartments can lead to premature diagnostic closure and delayed treatment.
3
Select appropriate initial investigations to evaluate all potential etiologies.
Order a bladder ultrasound (postrenal), urinalysis with microscopy (intrarenal), and urine electrolytes to calculate FENa (prerenal vs intrarenal).
An initial AKI workup must broadly and non-invasively assess all three categories simultaneously to quickly establish the exact etiology and guide management.

Key Concept

The initial evaluation of undifferentiated acute kidney injury requires concurrent, non-invasive assessment of prerenal, intrarenal, and postrenal etiologies using urine chemistry, urinalysis with microscopy, and bladder or renal ultrasound.
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Diagnostic Workup and Test Sequencing

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