Community-Acquired Pneumonia (CAP) is an acute infection of the pulmonary parenchyma accompanied by symptoms of acute illness and abnormal chest findings.
It is a lower respiratory tract infection acquired in the community within 24 hours to <2 weeks or occurring ≤48 hours of hospital admission in patients who do not meet the criteria for healthcare-associated pneumonia.
It occurs at the highest rates in the very young and the very old.
Potentially life-threatening especially in older adults and those with comorbid disease.


  • All patients with suspected community-acquired pneumonia (CAP) should have a chest radiograph (chest X-ray) to establish the diagnosis and identify complications (eg pleural effusion, multilobar disease)
  • Definite diagnosis of CAP must be based on a combination of clinical features and a chest X-ray showing lung shadowing that is likely to be new and not of other causes
  • Combining history and physical exam findings may be used to presumptively identify patients w/ pneumonia in cases where chest X-ray might not be easy to perform


Low-Risk Community-Acquired Pneumonia (CAP) 

  • Patients with stable vital signs, associated with low morbidity and mortality rate of <5%
  • Suitable for outpatient care
  • Patients with stable comorbid conditions [eg controlled diabetes mellitus (DM), coronary artery disease (CAD), renal insufficiency, chronic obstructive pulmonary disease (COPD), chronic liver disease, chronic alcohol abuse], if they may be treated as outpatients with a reasonable assurance for follow-up
  • Hospitalization of patients with low-risk CAP may be required in the following cases:
    • Complications of pneumonia itself
    • Exacerbation of underlying diseases
    • Inability to reliably take oral medications or receive outpatient care
    • Inability to maintain oral intake, uncontrollable vomiting, cognitive dysfunction
    • Social problems (eg unavailability of a caregiver, homelessness)
  • Vital Signs:
    • RR <30 breaths/minute
    • HR <125 beats/minute
    • Temperature >36°C or <40°C
    • Systolic BP >90 mmHg
    • Diastolic BP >60 mmHg
  • Clinical Features:
    • No altered mental status
    • No suspected aspiration
    • With no or has stable comorbid condition
    • Chest X-ray: Localized infiltrates and without pleural effusion

Moderate-Risk Community-Acquired Pneumonia (CAP) 

  • Associated with a complicated outcome and higher mortality rate
  • In-hospital parenteral therapy is recommended
  • Vital Signs:
    • RR ≥30 breaths/minute
    • HR ≥125 beats/minute
    • Temperature ≤36°C or ≥40°C
    • Systolic BP <90 mmHg
    • Diastolic BP ≤60 mmHg
  • Clinical Features:
    • Altered mental state of acute onset
    • Suspected aspiration
    • Extrapulmonary evidence of sepsis
    • Unstable or comorbid conditions (eg uncontrolled DM, active malignancies, CHF Class II-IV, COPD in acute exacerbation, decompensated liver disease)
  • Radiographic Findings:
    • Bilateral or multilobar involvement
    • Progression of lesion to 50% of initial finding within 24 hours, pleural effusion or abscess

High-Risk Community-Acquired Pneumonia (CAP) 

  • Associated with mortality rate of 36%
  • Warrants admission in the intensive care unit (ICU)
  • Clinical Features:
    • Any of the clinical features of moderate risk CAP plus any of the following:
      • Severe sepsis 
      • Septic shock 
      • Need for mechanical ventilation 
  • Risk factors for infection with P aeruginosa:
    • Chronic (>7 days within the past month) use of broad-spectrum antibiotic therapy
    • Bronchiectasis
    • Malnutrition
    • Steroid therapy


CRB65 Score for Mortality Risk Assessment

  • May be used as a prognostic baseline for patients with community-acquired pneumonia (CAP)

Prognostic Features (1 point for each feature present)

  • Confusion (abbreviated Mental Test score <8 or new disorientation)
  • Increased blood urea nitrogen (>7 mmol/L) (*CURB65; for hospitalized patients only)
  • Increased respiratory rate (RR) [≥30 breaths/minute]
  • Decreased blood pressure (BP) [<90/≤60 mmHg]
  • ≥65 years old


  • 0: Low risk (<1% mortality risk)
  • 1-2: Moderate risk (1-10% mortality risk)
    • Other features include extrapulmonary evidence of sepsis, suspected aspiration, unstable comorbid conditions, chest X-ray findings of multilobar involvement, pleural effusion abscess or progression of lesion to >50% of initial within 24 hours, & urea >7 mmol/L
  • 3-4: High risk (>10% mortality risk)
    • Other features include shock or signs of hypoperfusion (ie hypotension, altered mental state, urine output <30 mL/hour) and acute hypercapnea (PaCO2 >50 mmHg)

Pneumonia Severity Index (PSI)

  • Adopted by the American Thoracic Society, used to classify patients accordingly to mortality risk & predicts the need for hospitalization
  • Parameters include (step 1) age, comorbidities, physical exam, & (step 2) gender, laboratory & imaging findings

Risk Class

  • I: When no predictors are present
  • II: ≤70 points; mortality estimated at 0.6%
  • III: 70-90 points; mortality estimated at 0.9%
  • IV: 91-130 points; mortality estimated at 9.3%
  • V: >130 points; mortality estimated at 27%

Laboratory Tests

For Inpatients

Pulse Oximetry

  • Hypoxemia may be present in patients who do not have obvious signs of pneumonia
  • If SaO2 <92%, do an arterial blood gas measurement

Complete Blood Count

  • White blood cell (WBC) count >15 x 109/L strongly suggests a bacterial etiology
  • WBC count >20 x 109/L or <4 x 109/L is associated with increased mortality

Blood Chemistry

  • Urea, electrolytes, liver function tests (LFTs) should be requested to determine disease severity and identify associated liver or kidney disease
  • A baseline C-reactive protein (CRP) may be obtained for assessment of response to therapy 48-72 hours after admission

Determination of Causative Organism

Rationale for Testing to Determine Causative Organism

  • Results of diagnostic testing is anticipated to result in a change in patient’s antibiotic regimen (eg if unusual pathogens are suspected)
  • When causative organism may be of public health interest [eg severe acute respiratory syndrome (SARS) virus]
  • To allow antibiotic de-escalation and narrow the treatment regimen to prevent the emergence of resistance
  • Tests to determine microbial etiology are optional in patients w/ low-risk community-acquired pneumonia (CAP)

Blood Culture

  • Get at least 2 sets of pretreatment blood cultures
  • Although of low sensitivity, a positive blood culture is specific and is considered the gold standard in the etiologic diagnosis of pneumonia
  • High-risk CAP is the strongest indication for blood cultures because of the higher yield, the greater possibility of the presence of pathogens not covered by the usual empirical antibiotic therapy and the increased potential to affect antibiotic therapy

Sputum/Tracheal Aspirate Gram Stain and Culture

  • Obtain samples before start of antibiotic treatment
  • For intubated patients, an endotracheal aspirate sample should be obtained
  • Gram stain is quick and inexpensive, can assess quality of samples, help interpret culture results and may give an early indication of possible etiology
  • Culture for Legionella sp on specific media should be requested in areas where Legionella sp is endemic and in patients with recent travel


  • Antibody tests for the atypical pathogens [eg M pneumoniae, Chlamydia spp, C burnetii, influenza virus, adenovirus, respiratory syncytial virus (RSV), L pneumophila]
  • A single serum sample taken within 7 days of onset of symptoms is of limited value

Antigen Tests

  • Urine antigen tests for S pneumoniae and Legionella sp
    • Rapid, simple
    • Results not affected by antibiotic therapy
  • Rapid antigen tests for influenza


Chest Radiography (Chest X-ray) 

  • Useful for determining severity of disease & presence of complications
  • Typically shows lobar consolidation
  • May also show bilateral, more diffuse infiltrates
  • May suggest possible etiology, reveal associated conditions & assist in differentiating pneumonia from other conditions that may present similarly
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