What antibiotics are suitable for a patient with a confirmed penicillin allergy, presenting with a skin infection and bronchitis?

Antibiotic Selection for Penicillin-Allergic Patients with Skin Infection and Bronchitis

For a patient with confirmed penicillin allergy presenting with both skin infection and bronchitis, use doxycycline 100 mg orally twice daily as first-line therapy, as it effectively covers both conditions without cross-reactivity risk. 1

Initial Assessment of Penicillin Allergy

Before selecting antibiotics, document the specific details of the penicillin allergy, including reaction type (immediate versus delayed), severity, and timing. 2, 3 This is critical because approximately 90% of patients reporting penicillin allergy are not truly allergic when formally tested. 4

Key distinction: Immediate-type reactions (urticaria, angioedema, anaphylaxis, bronchospasm) occurring within hours suggest IgE-mediated allergy, while delayed reactions (rashes appearing days later) suggest different mechanisms. 4

Primary Antibiotic Recommendation

Doxycycline is the optimal single agent for this dual infection scenario:

• Dosing: 100 mg orally twice daily, with the first day potentially using 200 mg (100 mg every 12 hours) as a loading dose. 1
• Coverage: Effectively treats both skin/soft tissue infections and respiratory tract infections including bronchitis. 1
• Safety: Zero cross-reactivity with penicillins, making it safe regardless of allergy severity or timing. 3, 1
• Administration: Can be given with food or milk to reduce gastric irritation without affecting absorption. 1

Alternative Options Based on Infection Severity

For Mild to Moderate Infections:

Macrolides (azithromycin 500 mg day 1, then 250 mg daily for 4 days; or clarithromycin 250-500 mg twice daily) can cover both respiratory and skin infections, though bacterial failure rates of 20-25% are possible and resistance rates are 5-8% in most US areas. 2, 3

Trimethoprim-sulfamethoxazole is effective for skin/soft tissue infections but provides limited coverage for bronchitis. 2

For Severe Infections Requiring Beta-Lactam Coverage:

Cephalosporins with dissimilar side chains (ceftriaxone, cefazolin, cefuroxime, cefepime) can be used safely with only 1-2% cross-reactivity risk, regardless of the severity or timing of the original penicillin reaction. 2, 5

Critical caveat: Avoid cephalexin, cefaclor, and cefamandole in patients with immediate-type penicillin allergies, as these share R1 side chains with amoxicillin and ampicillin, creating documented cross-reactivity risk. 3, 5

Fluoroquinolones (levofloxacin 500-750 mg daily) provide broad-spectrum coverage for both conditions. 6 However, reserve these for more severe infections given concerns about resistance and adverse effects.

Carbapenems can be used without prior testing regardless of reaction severity or timing, with zero cross-reactivity concerns. 2

Clinical Decision Algorithm

1. Confirm true penicillin allergy: If the patient has only a vague history and time permits, consider penicillin skin testing, as 90% will test negative and can safely receive penicillins. 4

2. For confirmed immediate-type allergy (especially if recent <5 years):

   • First choice: Doxycycline 100 mg twice daily 1
   • Alternative: Azithromycin or clarithromycin 3
   • Avoid: All penicillins, cephalexin, cefaclor, cefamandole 2, 3, 5

3. For confirmed delayed-type allergy:

   • First choice: Doxycycline 100 mg twice daily 1
   • Alternative: Ceftriaxone or other dissimilar-side-chain cephalosporins 5
   • Avoid: Cephalexin if reaction was recent 5

4. For severe infections requiring beta-lactam:

   • Use ceftriaxone or cefazolin (only 1-2% cross-reactivity) 2, 5
   • Consider fluoroquinolones as alternative 6

Common Pitfalls to Avoid

Do not use cephalexin in patients with immediate-type penicillin allergies, as it shares side chains with common penicillins and has documented cross-reactivity. 3, 5 This is a frequent error in clinical practice.

Avoid macrolides with QT-prolonging drugs or in patients taking CYP3A4 inhibitors, as erythromycin and clarithromycin can cause dangerous cardiac arrhythmias. 3

Do not assume all cephalosporins are equally safe: Only those with dissimilar side chains (ceftriaxone, cefazolin, cefuroxime, cefepime) have the low 1-2% cross-reactivity rate. 2, 5

Duration of Therapy

For bronchitis combined with skin infection, treat for 7-10 days with doxycycline, ensuring adequate fluid intake to reduce esophageal irritation risk. 1 If using macrolides, azithromycin's shorter 5-day course may improve compliance. 3