This patient should begin empiric antibiotic therapy with moxifloxacin. He has community-acquired pneumonia (CAP) confirmed by chest radiograph. Because he is lacking other risk factors or associated comorbid conditions that would make other pathogens more likely, his CAP infection is probably caused by Streptococcus pneumoniae. The possibility of drug-resistant S. pneumoniae (DRSP) should be assessed because it will determine which agents are appropriate for empiric treatment. Factors that increase the risk for DRSP include age greater than 65 years, alcoholism, immunosuppression, certain medical comorbidities (COPD, chronic liver or kidney disease, cancer, diabetes, functional or anatomic asplenia, chronic heart disease), and recent (within the past 3-6 months) antimicrobial therapy with a β-lactam, macrolide, or fluoroquinolone antibiotic. Recent β-lactam use is associated with an increased risk for β-lactam resistance, while recent use of macrolides or fluoroquinolones is associated with pneumococcal resistance to the same class of antibiotic. Because this patient received treatment with a β-lactam within the past 3 months, he is at increased risk for DRSP. Recommended empiric therapy for patients with possible DRSP includes a respiratory fluoroquinolone (such as moxifloxacin) or a β-lactam plus a macrolide or doxycycline; however, because he was recently treated with a β-lactam antibiotic, treatment with a respiratory fluoroquinolone would be preferable.

Combination therapy with a β-lactam plus a macrolide or doxycycline is recommended, rather than monotherapy with a β-lactam, to provide coverage for atypical pathogens (such as Mycoplasma species), which are common causes of CAP. However, this therapy is not optimal for this patient with recent β-lactam use.

Patients with CAP who are otherwise healthy and without risk factors for DRSP are eligible for monotherapy with a macrolide (such as azithromycin) or doxycycline. However, this patient's recent use of antibiotics and subsequent increased risk for DRSP make either of these agents an inappropriate therapeutic choice.