Community-Acquired Pneumonia — Adult

Key Points

• Confirm a compatible syndrome and, when feasible, a new infiltrate before labeling the illness bacterial CAP.
• Use PSI plus clinical judgment to support site-of-care decisions.
• Do not use the old health care-associated pneumonia category to justify automatic broad-spectrum therapy.
• MRSA and Pseudomonas coverage should be based on validated individual risk factors and local epidemiology.
• Do not withhold initial antibiotics solely because procalcitonin is low when CAP is clinically and radiographically present.
• Routine anaerobic coverage is not required for aspiration pneumonia unless abscess or empyema is suspected.
• Treat influenza when indicated and account for viral-bacterial coinfection in severe disease.
• Reassess clinical stability and complications before deciding antibiotic duration or discharge.

Algorithm

1. Assess oxygenation, work of breathing, hemodynamics, mental status, and sepsis features.
2. Obtain chest imaging and targeted testing; calculate PSI or another validated severity tool when disposition is uncertain.
3. Identify severe CAP criteria and specific MRSA or Pseudomonas risk factors.
4. Start empiric antibiotics matched to outpatient, nonsevere inpatient, or severe inpatient status.
5. Add viral testing and antivirals when epidemiologically appropriate; obtain cultures when severe CAP or resistant pathogens are a concern.
6. Provide oxygen, fluids if hypoperfused, bronchodilators only for coexisting bronchospasm, and VTE prophylaxis for admitted patients.
7. Review cultures and clinical trajectory at 48 to 72 hours; narrow therapy when possible and investigate nonresponse.
8. Discharge when clinically stable with adequate oxygenation, oral intake, medication access, and follow-up.

Clinical Synopsis & Reasoning

Community-acquired pneumonia commonly presents with cough, sputum, dyspnea, pleuritic pain, fever, tachypnea, hypoxemia, or confusion, but older and immunocompromised adults may have subtle symptoms. The diagnosis is strengthened by a new pulmonary infiltrate on chest imaging in the setting of a compatible clinical syndrome. Chest radiography is usually sufficient, while CT is reserved for discordant cases, complications, or an alternative diagnosis. Assess severity before choosing antibiotics. The Pneumonia Severity Index is preferred by ATS/IDSA to support hospitalization decisions, while clinical judgment remains essential. Severe CAP is defined by major criteria such as need for mechanical ventilation or septic shock requiring vasopressors, or by multiple validated minor criteria. Routine blood and sputum cultures are not needed for most outpatients or nonsevere inpatients, but cultures are appropriate in severe CAP and when empiric MRSA or Pseudomonas coverage is being considered. Procalcitonin should not be used to withhold initial antibiotics when radiographically confirmed CAP is clinically suspected. Viral testing is useful during influenza, SARS-CoV-2, and other respiratory-virus circulation because a viral diagnosis can change isolation and antiviral decisions, although bacterial coinfection may still occur. Consider pulmonary embolism, heart failure, COPD or asthma exacerbation, aspiration pneumonitis, malignancy, tuberculosis, and noninfectious inflammatory lung disease when the presentation is atypical or response is poor.

Treatment Strategy & Disposition

Treat promptly with a regimen matched to setting and comorbidity. Healthy outpatients without major comorbidities can often receive high-dose amoxicillin or doxycycline, with macrolide monotherapy reserved for communities where pneumococcal macrolide resistance is sufficiently low. Outpatients with significant chronic heart, lung, liver, or kidney disease, diabetes, alcoholism, malignancy, or asplenia generally need broader therapy, such as amoxicillin-clavulanate or a cephalosporin combined with a macrolide or doxycycline, or a respiratory fluoroquinolone when appropriate. Nonsevere inpatients are commonly treated with a beta-lactam plus macrolide or a respiratory fluoroquinolone. Severe CAP generally warrants a beta-lactam plus macrolide or beta-lactam plus respiratory fluoroquinolone, with MRSA or antipseudomonal therapy only when validated risk factors are present. Do not add anaerobic coverage for suspected aspiration pneumonia unless lung abscess or empyema is suspected. Give influenza antiviral therapy promptly when influenza is confirmed or strongly suspected in a hospitalized patient. Support oxygenation, fluids when needed, venous thromboembolism prevention, and early mobilization. Most uncomplicated CAP can be treated for a minimum of about 5 days if the patient is clinically stable, afebrile, and improving, but duration should be individualized for resistant pathogens, complications, or slow response. Reassess patients who fail to improve by 48 to 72 hours for resistant organisms, empyema, abscess, obstruction, or an alternative diagnosis.

Epidemiology / Risk Factors

• CAP remains a major cause of hospitalization and death, especially in older adults and patients with chronic cardiopulmonary disease.
• Streptococcus pneumoniae remains important, while respiratory viruses account for a substantial proportion of adult pneumonia.
• Comorbid COPD, heart failure, diabetes, chronic kidney or liver disease, malignancy, and impaired immunity increase severity risk.
• Prior respiratory isolation of MRSA or Pseudomonas is a strong clue for recurrence and should influence empiric therapy.
• Vaccination against pneumococcus, influenza, COVID-19, and other indicated respiratory pathogens reduces preventable disease burden.

Investigations

Common Outpatient Regimens

Pharmacology

Prognosis / Complications

• Most otherwise healthy outpatients recover uneventfully with appropriate therapy.
• Mortality rises substantially with septic shock, mechanical ventilation, multilobar disease, severe hypoxemia, and major comorbidity.
• Clinical improvement often begins within 48 to 72 hours, while cough and fatigue may persist for weeks.
• Empyema, lung abscess, acute respiratory failure, sepsis, and decompensation of chronic disease drive prolonged hospitalization.
• Early mobilization, vaccination, smoking cessation, and optimized chronic disease control reduce future respiratory risk.

Patient Education / Counseling

• Take antibiotics exactly as prescribed and seek reassessment if breathing worsens, confusion develops, fever persists, or oral intake becomes inadequate.
• Explain that cough and fatigue may resolve more slowly than fever and oxygenation.
• Avoid smoking and vaping during recovery and offer cessation support.
• Review recommended influenza, pneumococcal, COVID-19, and other age- or risk-based vaccinations.
• Advise follow-up when symptoms fail to improve as expected or when persistent focal symptoms raise concern for obstruction or another diagnosis.

Notes

Antibiotic choice should be deliberately narrow enough to avoid unnecessary toxicity and resistance while still covering the pathogens that matter for that patient. Prior MRSA or Pseudomonas isolation, recent hospitalization with IV antibiotics, severe disease, and local epidemiology are more useful than the obsolete health care-associated pneumonia label. Disposition should integrate physiologic severity, ability to take oral medication, oxygen need, mental status, social support, and the risk of rapid deterioration. Clinical stability before discharge includes improving vital signs, adequate oral intake, stable mentation, and acceptable oxygenation. A patient who is not improving after 48 to 72 hours deserves a diagnostic reassessment rather than automatic antibiotic escalation.

References

1. ATS/IDSA Guidelines for Diagnosis and Treatment of Adults with Community-Acquired Pneumonia — Link