What is the recommended treatment for a 13-week pregnant woman with a Serratia marcescens urinary tract infection (UTI) and a penicillin allergy?

Treatment of Serratia marcescens UTI in a 13-Week Pregnant Woman with Penicillin Allergy

For a 13-week pregnant woman with Serratia marcescens UTI and penicillin allergy, aztreonam is the safest and most effective treatment option.

Assessment of Penicillin Allergy Status

First, determine the severity of the penicillin allergy:

• Low risk for anaphylaxis: No history of anaphylaxis, angioedema, respiratory distress, or urticaria following penicillin or cephalosporin administration
• High risk for anaphylaxis: History of anaphylaxis, angioedema, respiratory distress, or urticaria following penicillin or cephalosporin administration

Treatment Algorithm

First-line Treatment

• Aztreonam (1-2g IV every 8 hours or 500mg-1g IV every 6-8 hours) 1
  • FDA-approved for UTIs caused by Serratia marcescens
  • Safe in pregnancy
  • No cross-reactivity with penicillin allergy
  • Excellent gram-negative coverage

Alternative Options Based on Susceptibility Testing

If susceptibility testing is available, consider:

1. For patients at low risk for anaphylaxis:

   • Cefepime (1-2g IV every 12 hours) 2
   • Note: Cross-reactivity between penicillins and cephalosporins occurs in only about 10% of patients 3

2. For patients at high risk for anaphylaxis:

   • Aminoglycosides (e.g., amikacin) - based on susceptibility testing 4
   • Consider lower doses and monitoring levels due to pregnancy

Duration of Treatment

• Uncomplicated UTI: 7-10 days
• Complicated UTI: 10-14 days

Important Considerations

Pregnancy-Specific Concerns

• Avoid fluoroquinolones and tetracyclines due to potential fetal risks
• Monitor renal function and drug levels if aminoglycosides are used
• Consider hospitalization for IV therapy if symptoms are severe

Serratia marcescens Characteristics

• Often resistant to multiple antibiotics including ampicillin, first-generation cephalosporins, and nitrofurantoin 4
• Susceptibility to carbapenems and aminoglycosides is typically preserved 4

Follow-up

• Obtain urine culture 1-2 weeks after completing therapy to confirm clearance
• Monitor for recurrence throughout pregnancy
• Consider monthly urine cultures for the remainder of pregnancy

Pitfalls to Avoid

1. Do not use nitrofurantoin for Serratia marcescens (intrinsic resistance)
2. Do not use ampicillin or first-generation cephalosporins (high resistance rates)
3. Do not assume all cephalosporins are contraindicated in penicillin allergy (only ~10% cross-reactivity)
4. Do not delay treatment while waiting for susceptibility testing if the patient is symptomatic

Aztreonam represents the safest option for treating Serratia marcescens UTI in a pregnant woman with penicillin allergy, as it provides effective coverage against the organism without cross-reactivity with penicillin allergy.