A 64-year-old man with a history of alcohol-related cirrhosis and major depressive disorde… — USMLE Step 2 CK

A $64$ -year-old man with a history of alcohol-related cirrhosis and major depressive disorder is hospitalized for worsening abdominal distension and fever. His home medications include spironolactone, furosemide, lactulose, and citalopram. On admission, his temperature is $38.1$ °C ( $100.6$ °F) and blood pressure is $105/65$ mmHg. Physical examination reveals shifting dullness and trace bilateral pedal edema.

Diagnostic paracentesis yields ascitic fluid with a leukocyte count of $1,100$ /mm³ ( $85\%$ neutrophils). Arterial blood gas analysis on room air reveals:
pH: $7.29$
pCO $_2$ : $29$ mmHg
HCO $_3^-$ : $14$ mEq/L

The patient is empirically started on intravenous piperacillin-tazobactam and vancomycin, and he receives intravenous albumin on hospital day 1. On hospital day 3, his temperature has decreased to $37.1$ °C ( $98.8$ °F) and his abdominal pain has resolved. The ascitic fluid culture from admission grows *Escherichia coli* that is resistant to ampicillin but susceptible to ceftriaxone, cefepime, piperacillin-tazobactam, and ciprofloxacin. Blood cultures show no growth.

Which of the following is the most appropriate next step in management?

Discontinue the current antibiotics and begin intravenous ceftriaxoneAnswer
B
Continue the current regimen of intravenous piperacillin-tazobactam and vancomycin
C
Discontinue vancomycin and continue intravenous piperacillin-tazobactam
D
Administer an intravenous sodium bicarbonate infusion
E
Discontinue the current antibiotics and begin intravenous linezolid and ciprofloxacin

Answer

Discontinue the current antibiotics and begin intravenous ceftriaxone

The correct action is to discontinue the current antibiotics and begin intravenous ceftriaxone. This patient was appropriately started on broad-spectrum empiric antibiotics for severe Spontaneous Bacterial Peritonitis (SBP). Once culture data returns showing a single organism (*E. coli*) that is susceptible to a third-generation cephalosporin, principles of antibiotic stewardship dictate that therapy must be de-escalated to the most targeted, effective, and narrow-spectrum agent available. Ceftriaxone provides excellent ascitic fluid penetration and is the standard targeted therapy for susceptible SBP.

Step-by-Step Solution

Diagnose the primary condition and evaluate clinical trajectory.

The patient presented with Spontaneous Bacterial Peritonitis (SBP), evidenced by ascitic fluid neutrophils

>250

/mm³. By hospital day 3, the infection is clinically resolving.

Understanding the clinical status is required before modifying empiric life-saving therapies.

Interpret the culture and sensitivity data to practice antibiotic stewardship.

The culture grew *E. coli* susceptible to ceftriaxone. The current empiric regimen (piperacillin-tazobactam and vancomycin) is overly broad and should be narrowed.

De-escalating to the narrowest effective agent (ceftriaxone) reduces the risk of multi-drug resistant organism selection and antibiotic-associated complications.

Evaluate the arterial blood gas using Winter's formula to rule out secondary acid-base disorders.

Expected pCO

_2

1.5 \times [\text{HCO}_3^-] + 8 \pm 2

1.5 \times 14 + 8 = 29 \pm 2

. The measured pCO

_2

29

mmHg, confirming a fully compensated metabolic acidosis.

This rules out the need for aggressive acid-base interventions like sodium bicarbonate infusions, which are reserved for severe, uncompensated acidosis or specific toxicities.

Review the patient's home medication list for potential drug-drug interactions.

The patient takes citalopram (an SSRI). This strictly contraindicates the use of MAOIs or agents with MAOI activity, such as linezolid.

Overlooking home psychiatric medications when prescribing antibiotics can lead to fatal complications like serotonin syndrome.

Key Concept

Antibiotic Stewardship in Spontaneous Bacterial Peritonitis

Estimated Time:1m 30s

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