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Difficulty: MediumInterpreting Laboratory Results

Match each patient's clinical presentation and associated laboratory profile with the most appropriate diagnostic category of acute kidney injury.

  • A 65-year-old man with 3 days of intractable vomiting; BUN 45 mg/dL, Cr 1.2 mg/dL. Urinalysis shows a specific gravity of 1.025 and bland sediment.Prerenal azotemia
  • A 58-year-old woman receiving intravenous gentamicin for bacteremia; BUN 30 mg/dL, Cr 2.5 mg/dL. Urinalysis shows muddy brown granular casts.Acute tubular necrosis
  • A 42-year-old man presenting with a new maculopapular rash after starting omeprazole; BUN 25 mg/dL, Cr 2.1 mg/dL. Urinalysis shows WBCs and WBC casts.Acute interstitial nephritis
  • A 30-year-old man presenting with gross hematuria following a respiratory infection; BUN 28 mg/dL, Cr 1.9 mg/dL. Urinalysis shows 2+ protein and RBC casts.Acute glomerulonephritis

Answer

The correct matching pairs are: 1) Vomiting and bland sediment matches with Prerenal azotemia; 2) Gentamicin and muddy brown casts matches with Acute tubular necrosis; 3) Omeprazole, rash, and WBC casts matches with Acute interstitial nephritis; 4) Post-URI hematuria and RBC casts matches with Acute glomerulonephritis.
Prerenal azotemia is driven by hypovolemia, resulting in a high BUN:creatinine ratio and a concentrated, bland urine sediment. Acute tubular necrosis (ATN) is frequently caused by nephrotoxins like aminoglycosides, leading to epithelial cell death and pathognomonic muddy brown granular casts. Acute interstitial nephritis (AIN) is a drug-induced allergic reaction (e.g., PPIs) marked by WBCs and WBC casts alongside systemic signs like rash. Acute glomerulonephritis involves glomerular capillary inflammation, allowing RBCs to leak into the tubules and form dysmorphic RBCs and RBC casts.

Step-by-Step Solution

1
Analyze the clinical history and BUN/creatinine ratio for evidence of hypoperfusion.
Patient 1 has a clear history of fluid loss (vomiting) and a BUN:Cr ratio > 20:1, indicating intact tubular function trying to conserve volume.
BUN is reabsorbed alongside sodium and water in the proximal tubule; in low-flow states, disproportionate BUN elevation occurs, pointing to a prerenal cause.
2
Evaluate the urinalysis sediment findings for specific cast elements.
Muddy brown casts are found in Patient 2, WBC casts in Patient 3, and RBC casts in Patient 4.
Urine sediment acts as a 'liquid biopsy'. Casts form within the renal tubules and directly reflect the underlying pathology (tubular cell death, interstitial inflammation, or glomerular bleeding).
3
Match the combined clinical and laboratory profile to the correct pathophysiologic category of acute kidney injury.
The distinct laboratory profiles successfully map to Prerenal azotemia, Acute tubular necrosis, Acute interstitial nephritis, and Acute glomerulonephritis.
Establishing the precise etiology of AKI through laboratory interpretation is critical for guiding specific management (e.g., volume repletion, stopping offending drugs, or specific immunosuppression).

Key Concept

Interpretation of urinary sediment and BUN-to-creatinine ratio is essential for differentiating the primary mechanisms of acute kidney injury: prerenal, intrinsic tubular, interstitial, and glomerular disease.
Estimated Time:1m 0s
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Interpreting Laboratory Results

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