Community-Acquired Pneumonia

GENERAL PRINCIPLES

• The predominant organism involved is S. pneumoniae; other bacterial etiologies are H. influenzae and M. catarrhalis. Pneumonia caused by atypical agents, such as Legionella pneumophila, C.

• Community-acquired MRSA is an important cause of severe, necrotizing pneumonia.

• Patients aged 65 years or older, and those with certain medical conditions, should receive the pneumococcal vaccination with both the 23-valent and the 13-valent vaccine, as recommended per CDC guidelines.²⁰

DIAGNOSIS

Clinical Presentation

• The presentation of CAP is extremely variable. Fever and respiratory symptoms, including cough with sputum production, dyspnea, and pleuritic chest pain, are common in immunocompetent patients. Signs include tachypnea, rales, or evidence of consolidation on auscultation.

• CAP presents acutely, over a matter of hours to days. If a patient has symptoms for more than 2-3 weeks, particularly if accompanied by weight loss or night sweats, this should raise the question of an alternate diagnosis, such as mycobacterial or fungal infection.

Diagnostic Testing

• Prior to antibiotic therapy, sputum Gram stain and culture of an adequate sputum sample and blood cultures before antibiotic therapy should be obtained in all patients who are going to be hospitalized, and, if disease is severe, urinary antigen tests for S. pneumoniae and L. pneumophila.

• Nasopharyngeal swab for influenza, SARS-CoV-2, or other virus detection by PCR, and respiratory samples for atypical pathogens should be sent in selected cases.

• If TB is suspected, sputum for acid-fast stain and culture should be obtained, and the patient should be placed on airborne isolation.

• Chest radiography should be performed and may reveal lobar consolidation, interstitial infiltrates, or cavitary lesions, confirming the diagnosis.

• Fiberoptic bronchoscopy may be used for detection of less common organisms, especially in immunocompromised patients, or if the patient is not responding to adequate therapy.

TREATMENT

• All patients should be assessed for hospitalization and evaluated for comorbid factors, oxygenation, and severity of illness using validated severity scales such as the Pneumonia Severity Index or CURB- 65.

• Antibiotics should be given as soon as CAP is diagnosed, ideally within 4 hours of arrival to the hospital, as delays lead to higher mortality. Antibiotic therapy should be narrowed once a specific microbiologic etiology has been identified.

• Empiric antibiotic therapy with an emphasis on targeting likely pathogens based on comorbidities and other risk factors is described in Table 14-9.²¹

TABLE 14-9

EMPIRIC TREATMENT FOR COMMUNITY-ACQUIRED PNEUMONIA

Outpatient Regimens

No comorbidities^a or risk factors for MRSA or Pseudomonas aeruginosa^b

• Monotherapy with amoxicillin 1000 mg PO q8 hours OR doxycycline 100 mg PO q12 hours for at least 5 d

• Monotherapy with azithromycin 500 mg PO on first day, then 250 mg qday on day 2-5

With comorbidities^a

• Combination therapy with amoxicillin/clavulanate 875 mg/125 mg PO q12 hours, cefpodoxime 200 mg PO q12 hours, OR cefuroxime 500 mg PO q12 hours for at least 5 d PLUS doxycycline or azithromycin

• Monotherapy with a respiratory fluoroquinolone (e.g., levofloxacin 750 mg PO qday, moxifloxacin 400 mg PO qday)

Inpatient Regimens

Nonsevere

• β-Lactam (ampicillin-sulbactam 1.5-3 g IV q6 hours, ceftriaxone 1-2 g IV qday, cefotaxime 1-2 g IV q8 hours, or ceftaroline 600 mg IV q12 hours) PLUS macrolide (e.g., azithromycin)

• Monotherapy with respiratory fluoroquinolone (e.g., levofloxacin, moxifloxacin)

Severe disease

• β-Lactam PLUS macrolide

• β-Lactam PLUS fluoroquinolone

Prior respiratory isolation of MRSA or P. aeruginosa

• MRSA: Expand empiric coverage with vancomycin 15 mg/kg IV q12 hours (adjusted based on levels and renal clearance) or linezolid 600 mg IV/PO q12 hours regardless of disease severity.

• P aeruginosa: Expand empiric coverage with piperacillin-tazobactam 4.5 g IV q6 hours, cefepime 2 g IV q8 hours, ceftazidime 2 g IV q8 hours, imipenem 500 mg IV q6 hours, meropenem 1 g IV q8 hours, or aztreonam 2 g IV q8 hours regardless of disease severity.

• Obtain cultures (and/or nasal PCR for MRSA, if available) to help tailor antibiotic therapy. Recent hospitalization and receipt of parenteral antibiotics in the preceding 90 d and locally validated risk factors for MRSA or P aeruginosa

• Nonsevere disease: Obtain cultures (and/or nasal PCR for MRSA, if available) but do not expand coverage unless MRSA and/or P aeruginosa is identified.

• Severe disease: Expand empiric coverage for MRSA and/or P aeruginosa and obtain cultures (and nasal PCR for MRSA, if available) to help tailor antibiotic therapy.

MRSA, methicillin-resistant Staphylococcus aureus; PCR, polymerase chain reaction.

Adapted from Metlay JP, Waterer GW, Long AC, et al. Diagnosis and treatment of adults with community-acquired pneumonia. An official clinical practice guideline of the American Thoracic Society and Infectious Diseases Society of America. Am J Respir Crit Care Med. 2019;200(7):e45-e67.

^aChronic heart, lung, liver, or renal disease; diabetes mellitus; alcoholism; malignancy; or asplenia.

^bPrior respiratory isolation of MRSA or P. aeruginosa or recent hospitalization AND receipt of parenteral antibiotics in the preceding 90 days.

• Thoracentesis of pleural effusions should be performed, with analysis of pH, cell count, Gram stain and bacterial culture, protein, and lactate dehydrogenase (LDH) (see Chapter 10, Pulmonary Diseases). Empyemas should be drained.