Key Concept: IV vancomycin for MRSA coverage in severe CF exacerbation (hypoxia, recurrent hospitalizations, skin abscesses). Doxycycline = oral, only for mild exacerbations, avoid <8yo. Nafcillin = MSSA only, not MRSA. Azithromycin = long-term maintenance, not acute treatment.
Antibiotic
Key Features
Vancomycin
Indicated when hypoxia present (O2 sat <90%)
IV route required for severe infection
Covers MRSA (methicillin-resistant S. aureus)
Indicated when recurrent hospitalizations suggest MRSA colonization
Indicated when history of skin abscesses requiring incision & drainage
Drug of choice for MRSA in severe CF exacerbation
Doxycycline
Appropriate for MILD CF exacerbation managed as outpatient
Oral route limits utility in severe disease
Generally avoided in children <8 years (permanent tooth staining)
Has MRSA activity, but only appropriate when severity does not mandate IV
Wrong choice when patient has hypoxia + retractions
Nafcillin
Covers methicillin-SENSITIVE S. aureus (MSSA) only
Does NOT cover MRSA — fails when MRSA is likely
IV beta-lactam (not oral)
Appropriate for MSSA bacteremia or pneumonia without MRSA risk factors
Azithromycin
Covers atypical organisms (Mycoplasma, Chlamydia)
NOT used for treatment of acute CF exacerbation
Used as chronic maintenance therapy in CF (anti-inflammatory, slows FEV1 decline)