Community-Acquired Pneumonia

A 68-year-old man with a history of diabetes mellitus, COPD, and chronic smoking presents with 4 days of fever, productive cough, pleuritic chest pain, and progressive shortness of breath. His family reports new-onset confusion since this morning. He has had no recent hospitalizations or antibiotic use. On examination, his temperature is 38.8°C, heart rate 112/min, respiratory rate 32/min, blood pressure 92/58 mm Hg, and oxygen saturation 88% on room air. Chest examination reveals dullness to percussion, bronchial breath sounds, and crackles over the right lower lung field. Laboratory studies show leukocytosis, elevated CRP, mild hyponatremia, and elevated blood urea nitrogen. A chest X-ray demonstrates right lower lobe consolidation. Diagnosis?

Diagnosis is Community-Acquired Pneumonia (CAP).

1. Definition

Community-acquired pneumonia is an acute infection of the lung parenchyma acquired outside the hospital setting, characterized by:

1.1 Diagnostic Features

1.      Symptoms or signs of lower respiratory tract infection

2.      New pulmonary infiltrate on imaging

3.      Supportive systemic or laboratory evidence of infection or inflammation

There is no single gold-standard diagnostic test. Clinical examination alone cannot reliably confirm CAP, and imaging is important for establishing the diagnosis.

1.2 Scope

The following CAP recommendations primarily apply to immunocompetent adults. Patients with substantial immunocompromise require separate diagnostic and therapeutic considerations because of their broader spectrum of potential pathogens and different management requirements.

2. Etiology

2.1 Common and Clinically Important Bacterial Pathogens

1.      Streptococcus pneumoniae

2.      Haemophilus influenzae

3.      Moraxella catarrhalis

4.      Staphylococcus aureus

5.      Mycoplasma pneumoniae

6.      Chlamydia pneumoniae

7.      Legionella pneumophila

MRSA, Pseudomonas aeruginosa, and resistant gram-negative organisms are less common and are mainly considered when specific validated risk factors are present.

2.2 Viral Causes

1.      Influenza

2.      Respiratory syncytial virus

3.      Rhinovirus

4.      Human metapneumovirus

5.      Parainfluenza viruses

6.      Coronaviruses, including SARS-CoV-2

Viral-bacterial coinfection can occur, especially in hospitalized and severely ill patients.

3. Pathophysiology

3.1 Route of Infection

1.      Most bacterial CAP develops through microaspiration of colonized oropharyngeal secretions

2.      Inhalation of infectious particles is important for respiratory viruses and some bacterial pathogens

3.2 Core Process

1.      Pathogens reach and proliferate within the lower respiratory tract

2.      Alveolar macrophages initiate an inflammatory response

3.      Cytokines and chemokines recruit neutrophils and other inflammatory cells

4.      Alveolar inflammation produces edema and protein-rich exudate

5.      Alveolar filling causes consolidation and impaired gas exchange

4. Physiological Consequences

1.      Ventilation-perfusion mismatch

2.      Hypoxemia

3.      Increased work of breathing

4.      Acute respiratory failure in severe disease

5.      Sepsis, septic shock, and multiorgan dysfunction in advanced disease

5. Classification and Severity

5.1 By Setting

1.      Community-acquired pneumonia

2.      Hospital-acquired pneumonia

3.      Ventilator-associated pneumonia

5.2 CURB-65

1.      Confusion

2.      Urea >7 mmol/L

3.      Respiratory rate ≥30/min

4.      Blood pressure <90 mm Hg systolic or ≤60 mm Hg diastolic

5.      Age ≥65 years

5.2.1 Interpretation

1.      0 to 1: usually low risk

2.      2: consider hospital assessment or admission

3.      ≥3: high-risk CAP and assessment for higher-level care

CURB-65 supports severity assessment but should not replace clinical judgment. The Pneumonia Severity Index (PSI) is preferred by ATS/IDSA when determining the need for hospitalization.

5.3 ATS/IDSA Severe CAP Criteria

5.3.1 Major Criteria

1.      Respiratory failure requiring invasive mechanical ventilation

2.      Septic shock requiring vasopressors

5.3.2 Minor Criteria

1.      Respiratory rate ≥30/min

2.      PaO₂/FiO₂ ≤250

3.      Multilobar infiltrates

4.      Confusion or disorientation

5.      BUN ≥20 mg/dL

6.      Leukocyte count <4,000/μL

7.      Platelet count <100,000/μL

8.      Core temperature <36°C

9.      Hypotension requiring aggressive fluid resuscitation

Severe CAP is defined by 1 major criterion or at least 3 minor criteria.

5.4 Case Severity

This patient has a CURB-65 score of at least 4 based on:

1.      Confusion

2.      Respiratory rate ≥30/min

3.      Diastolic blood pressure ≤60 mm Hg

4.      Age ≥65 years

The CURB-65 score would be 5 if serum urea is confirmed to be >7 mmol/L.

For ATS/IDSA severe CAP, he has two definite minor criteria:

1.      Respiratory rate ≥30/min

2.      Confusion

A third minor criterion cannot be confirmed from the information provided because the exact BUN and PaO₂/FiO₂ values are not given, and a blood pressure of 92/58 mm Hg alone does not meet the separate criterion of hypotension requiring aggressive fluid resuscitation.

Therefore, this patient clearly has high-risk CAP, but ATS/IDSA severe CAP cannot be definitively confirmed from the available data.

6. Clinical Features

6.1 Symptoms

1.      Cough, with or without sputum

2.      Dyspnea

3.      Pleuritic chest pain

4.      Fever or chills

5.      Fatigue and malaise

6.2 Signs

1.      Tachypnea

2.      Hypoxemia

3.      Crackles

4.      Bronchial breath sounds

5.      Dullness to percussion

6.      Egophony

7.      Tachycardia and hypotension in severe disease

6.3 Older Adults

Older adults may present with confusion, functional decline, weakness, or reduced oral intake, sometimes with less prominent respiratory symptoms.

7. Diagnosis

7.1 Laboratory Evaluation

1.      Leukocytosis, leukopenia, neutrophilia, or bandemia may occur

2.      CRP and procalcitonin may be elevated

3.      Laboratory abnormalities may assist severity assessment and identification of organ dysfunction

Procalcitonin should not be used alone to withhold initial antibacterial therapy when bacterial CAP is clinically suspected. Serial procalcitonin may contribute to antimicrobial stewardship in selected patients but should be interpreted with the clinical picture.

7.2 Imaging

1.      Chest radiography is standard first-line imaging

2.      Lung ultrasound is an acceptable alternative when appropriate expertise is available

3.      CT chest is more sensitive and is useful when the diagnosis remains uncertain, complications are suspected, or chest radiography is inconclusive

4.      Sonographic findings supporting pneumonia include subpleural consolidation and dynamic air bronchograms

7.3 Microbiology

Routine extensive microbiologic testing is not required for most low-risk outpatients.

1.      Blood cultures are recommended particularly in severe CAP and when MRSA or Pseudomonas coverage is being considered

2.      Respiratory cultures are recommended in severe CAP and when resistant pathogens are suspected

3.      Pneumococcal urinary antigen testing may be considered in severe CAP

4.      Legionella urinary antigen testing is appropriate in severe CAP or when epidemiologic factors such as an outbreak or recent travel are present

7.4 Viral Testing

1.      Test for influenza when it is circulating in the community

2.      Test for SARS-CoV-2 when clinically or epidemiologically appropriate

3.      Molecular respiratory viral testing may be useful in hospitalized or severe CAP when results could alter management

4.      A positive viral test does not exclude bacterial coinfection

8. Management

8.1 Empiric Antibacterial Therapy

8.1.1 Outpatient Without Major Comorbidities

1.      Amoxicillin

2.      Doxycycline

3.      Macrolide monotherapy only if local pneumococcal macrolide resistance is <25%

8.1.2 Outpatient With Comorbidities

Relevant comorbidities include chronic heart, lung, liver, or renal disease, diabetes mellitus, alcoholism, malignancy, and asplenia.

Options include:

1.      Amoxicillin-clavulanate or an appropriate oral cephalosporin plus a macrolide or doxycycline

2.      Respiratory fluoroquinolone monotherapy in appropriate patients

8.1.3 Hospitalized Nonsevere CAP

1.      Beta-lactam plus macrolide

2.      Respiratory fluoroquinolone monotherapy as an alternative in appropriate patients

8.1.4 Severe CAP

Preferred regimens include:

1.      Beta-lactam plus macrolide

2.      Beta-lactam plus respiratory fluoroquinolone

8.2 MRSA and Pseudomonas Coverage

Routine empiric MRSA or antipseudomonal therapy is not recommended without appropriate risk factors.

Important predictors include:

1.      Prior respiratory isolation of MRSA or Pseudomonas aeruginosa, particularly within the preceding year

2.      Recent hospitalization with parenteral antibiotic exposure within the previous 90 days

3.      Locally validated epidemiologic risk factors

Prior respiratory isolation of MRSA or Pseudomonas aeruginosa is a particularly strong predictor of subsequent infection with the same organism.

For nonsevere inpatient CAP:

1.      Prior respiratory isolation of MRSA or Pseudomonas supports empiric coverage for that organism while appropriate microbiologic testing is obtained

2.      When the principal concern is recent hospitalization with parenteral antibiotics, obtain appropriate microbiologic testing and consider local epidemiology and validated local risk factors rather than automatically providing broad-spectrum therapy

For severe CAP:

1.      Prior respiratory isolation of MRSA or Pseudomonas strongly supports empiric coverage for the corresponding organism

2.      Recent hospitalization with parenteral antibiotics within 90 days should be interpreted together with locally validated risk factors, previous microbiologic data, and local epidemiology when deciding whether empiric MRSA or antipseudomonal treatment is warranted

3.      Obtain appropriate cultures when expanded coverage is initiated

4.      De-escalate promptly when microbiologic testing is negative and the clinical course does not support infection with a resistant pathogen

The objective is to balance the risk of inadequate initial therapy against unnecessary broad-spectrum antibiotic exposure.

8.3 CAP With Positive Viral Testing

The 2025 ATS guideline and IDSA differ on some viral-positive CAP recommendations.

1.      In otherwise healthy outpatients without comorbidities, the ATS guideline suggests that empiric antibacterial therapy may be withheld when a respiratory virus is identified and bacterial coinfection is unlikely

2.      In severe CAP, empiric antibacterial therapy is appropriate because bacterial coinfection cannot be safely excluded

3.      The 2025 ATS guideline suggests empiric antibiotics for viral-positive outpatients with comorbidities and for viral-positive nonsevere inpatients

4.      IDSA did not endorse these two recommendations and favors individualized antibacterial treatment based on illness severity, clinical findings, radiographic findings, biomarkers, microbiology, comorbidities, and the estimated likelihood of bacterial coinfection

8.4 Anaerobic Coverage

Routine additional anaerobic coverage is not recommended for suspected aspiration pneumonia unless there is concern for:

1.      Lung abscess

2.      Empyema

3.      Another specific anaerobic infection

8.5 Antiviral Therapy

1.      Oseltamivir or another appropriate influenza antiviral should be given to hospitalized patients with influenza-positive CAP regardless of the duration of illness before diagnosis

2.      Antiviral treatment is also suggested for outpatients with influenza-positive CAP regardless of the duration of illness before diagnosis

3.      Appropriate pathogen-specific antiviral treatment should be used for other viral infections when indicated

8.6 Corticosteroids

1.      For adult inpatients with nonsevere CAP, routine systemic corticosteroids are not recommended in the absence of another established indication

2.      For adult inpatients with severe CAP, the 2025 ATS guideline suggests systemic corticosteroids

3.      This is a conditional recommendation based on low-quality evidence

4.      The recommendation does not apply to severe CAP caused by influenza

5.      Hyperglycemia is the best-established important adverse effect and should be monitored

6.      Factors that may argue against corticosteroid therapy or require individualized consideration include influenza, suspected or confirmed Aspergillus infection, uncontrolled diabetes mellitus, and recent gastrointestinal bleeding

7.      Corticosteroids may still be independently indicated for conditions such as COPD exacerbation, asthma, or refractory septic shock

The potential benefit of corticosteroids appears most relevant in appropriately selected patients with severe CAP, particularly when treatment is initiated early after severe disease develops.

8.7 Duration of Antibacterial Therapy

Duration should be guided by clinical stability, severity, pathogen, and complications.

1.      Clinically stable nonsevere CAP: treatment for less than 5 days may be appropriate, with a minimum effective duration of 3 days

2.      Severe CAP: treat for at least 5 days and until clinical stability is achieved

3.      Longer courses may be required for S. aureus, Pseudomonas, Legionella, bacteremia, lung abscess, empyema, necrotizing pneumonia, extrapulmonary infection, or inadequate clinical response

9. Acute High-Risk or Severe CAP

1.      Do not delay appropriate antimicrobial therapy in critically ill patients while awaiting diagnostic results

2.      Provide supplemental oxygen and ventilatory support according to respiratory status

3.      Evaluate and treat sepsis and septic shock

4.      Give intravenous fluids when indicated, with frequent reassessment

5.      Use vasopressors when hypotension persists despite adequate fluid resuscitation

6.      Consider ICU admission when severe CAP criteria or organ-support requirements are present

10. Prognosis and Monitoring

Prognosis depends on age, comorbidities, pathogen, physiologic derangement, and illness severity.

Monitor for:

1.      Respiratory failure

2.      Sepsis or septic shock

3.      Parapneumonic effusion or empyema

4.      Lung abscess or necrotizing pneumonia

5.      Acute kidney injury and other organ dysfunction

6.      Cardiovascular complications, including myocardial infarction, arrhythmias, and heart failure

Clinical stability generally includes improvement in temperature, heart rate, respiratory rate, blood pressure, oxygenation, mental status, and ability to maintain oral intake.

11. Long-Term Follow-Up

1.      In adults with CAP whose symptoms resolve within 5 to 7 days, routine follow-up chest imaging is not recommended

2.      Follow-up imaging should be considered when symptoms or radiographic abnormalities persist

3.      Imaging should also be performed when clinically indicated for evaluation of suspected underlying lung malignancy

4.      Patients who independently meet appropriate lung cancer screening criteria should undergo screening according to the relevant screening recommendations

5.      Address smoking cessation

6.      Ensure appropriate influenza, pneumococcal, COVID-19, and other indicated vaccinations

7.      Optimize underlying conditions such as COPD, diabetes mellitus, and cardiovascular disease

12. Key Clinical Insight

Fever + productive cough + dyspnea + focal chest findings + new pulmonary consolidation strongly support community-acquired pneumonia.

Confusion + tachypnea + hypotension + hypoxemia in an older patient indicate high-risk disease and require urgent assessment for severe CAP, sepsis, respiratory failure, and the need for organ support.

References

1.      Jones BE, Ramirez JA, Oren E, et al. Diagnosis and Management of Community-acquired Pneumonia: An Official American Thoracic Society Clinical Practice Guideline. Am J Respir Crit Care Med. 2026;212(1):24-44. Published online July 18, 2025. doi:10.1164/rccm.202507-1692ST.

2.      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. doi:10.1164/rccm.201908-1581ST.

3.      Klompas M, Al-Hasan M, Al Mohajer M, et al. Infectious Diseases Society of America Position Statement: Why IDSA Did Not Endorse the Community-Acquired Pneumonia Guidelines 2025 Update. Clin Infect Dis. 2026;82(4):622-624. doi:10.1093/cid/ciaf625.