Last updated: June 10, 2022

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Pneumonia is a respiratory infection characterized by inflammation of the alveolar space and/or the interstitial tissue of the lungs. In industrialized nations, it is the leading infectious cause of death. Pneumonia is most commonly transmitted via aspiration of airborne pathogens (primarily bacteria, but also viruses and fungi) but may also result from the aspiration of stomach contents. The most likely causal pathogens can be narrowed down based on patient age, immune status, and where the infection was acquired (community-acquired or hospital-acquired). Pneumonia is classified based on clinical features as either typical and atypical; each type has its own spectrum of commonly associated pathogens. Typical pneumonia manifests with sudden onset of malaise, fever, and a productive cough. On auscultation, crackles and bronchial breath sounds are audible. Atypical pneumonia manifests with gradual onset of unproductive cough, dyspnea, and extrapulmonary manifestations. Auscultation is usually unremarkable. Some patients may present with elements of both types. Diagnostics include blood tests for inflammatory parameters and pathogen detection in blood, urine, or sputum samples. Chest x-ray in cases of typical pneumonia shows opacity restricted to one lobe, while x-ray in atypical pneumonia may show diffuse, often subtle infiltrates. Together with the characteristic clinical features, newly developed pulmonary infiltrate on chest x-ray confirms the diagnosis. Management consists of empiric antibiotic treatment and supportive measures (e.g., oxygen administration, antipyretics).


Pneumonia pathogens according to location
Type of pneumonia Common pathogens
Lobar pneumonia
Interstitial pneumonia
Cryptogenic organizing pneumonia
  • Noninfectious
Pneumonia pathogens according to affected population
Type of pneumonia Common pathogens
Pneumonia in immunocompromised patients
Pneumonia in newborns
Pneumonia in children (4 weeks –18 years)
Pneumonia in young adults (18–40 years)
Pneumonia in adults (40–65 years)
Pneumonia in elderly individuals
Recurrent pneumonia

Track my respiration: chlassic strep formation”: C. trachomatis, Mycoplasma, Respiratory syncytial virus, Chlamydia pneumoniae, and Streptococcus pneumoniae are the most common causative agents of pneumonia in children.

Risk factors

Pneumonia can be classified according to etiology, location acquired, clinical features, and the area of the lung affected by the pathology.


Location acquired

  • Community-acquired pneumonia (CAP): pneumonia that is acquired outside of a healthcare establishment
  • Hospital-acquired pneumonia (HAP): nosocomial pneumonia, with onset > 48 hours after admission
  • Healthcare-associated pneumonia (HCAP): pneumonia that is acquired in healthcare facilities (e.g., hospital, nursing homes, hemodialysis centers, and outpatient clinics); this terminology is no longer recommended but is included for historical purposes. [9][10]

Clinical features

Area of lung affected by the pathology

Routes of infection


  1. Failure of protective pulmonary mechanisms; (e.g., cough reflex, mucociliary clearance , alveolar macrophages )
  2. Infiltration of the pulmonary parenchyma by the pathogen interstitial and alveolar inflammation
  3. Impaired alveolar ventilation ventilation/perfusion (V/Q) mismatch with intrapulmonary shunting (right to left)
  4. Hypoxia due to increased alveolar-arterial oxygen gradient

Pattern of involvement

  • Lobar pneumonia
    • Classic (typical) pneumonia of an entire lobe; primarily caused by pneumococci
    • Characterized by inflammatory intra-alveolar exudate, resulting in consolidation
    • Can involve the entire lobe or the whole lung
Stages of lobar pneumonia
Stages Macroscopic findings Microscopic findings
Congestion (day 1–2)
  • Alveolar lumens with serous exudate, bacteria, and rare inflammatory cells
Red hepatization (day 3–4)
  • Parenchymal consolidation
  • Red-brown
  • Dry and firm
  • Liver–like consistency
  • Reversible
Gray hepatization (day 5–7)
  • Uniformly gray
  • Liver–like consistency
Resolution (day 8 to week 4)
  • Gradual aeration of the affected segment

Typical pneumonia

Typical pneumonia is characterized by a sudden onset of symptoms caused by lobar infiltration.

Suspect bacterial pneumonia in immunocompromised patients with acute high fever and pleural effusion.

Atypical pneumonia

Atypical pneumonia typically has an indolent course (slow onset) and commonly manifests with extrapulmonary symptoms.

This classification does not have a major impact on patient management because it is not always possible to clearly distinguish between typical and atypical pneumonia.

Pneumonia is a clinical diagnosis based on history, physical examination, laboratory findings, and CXR findings. Consider microbiological studies and advanced diagnostics based on patient history, comorbidities, severity, and entity of pneumonia. [12]

Laboratory studies


Microbiological studies

Indication Microbiological studies to consider [9][10][12]
Any admitted patient
  • MRSA nares swab (PCR and/or culture)

Any patient being treated empirically for MRSA or P. aeruginosa

Severe CAP



Influenza season


Chest x-ray (posteroanterior and lateral)

A new pulmonary infiltrate on chest x-ray in a patient with classic symptoms of pneumonia confirms the diagnosis.

Typical pneumonia usually appears as lobar pneumonia on x-ray, while atypical pneumonia tends to appear as interstitial pneumonia. However, the underlying pathogen cannot be conclusively identified based on imaging results alone.

Chest CT (usually without contrast)

Advanced diagnostics


  • Indications
    • Suspected mass (e.g., recurrent pneumonia)
    • Need for pathohistological diagnosis (e.g., biopsy of a central mass discovered on CT)
    • Inconclusive results on CT
    • Poor response to treatment

Diagnostic thoracentesis


Criteria for hospitalization

Every patient should be assessed individually and clinical judgment is the most important factor. The pneumonia severity index (PSI) and the CURB-65 score are tools that can help to determine whether to admit a patient.

  • CURB-65 score [17]
    • Confusion (disorientation, impaired consciousness)
    • Serum Urea > 7 mmol/L (20 mg/dL)
    • Respiratory rate ≥ 30/min
    • Blood pressure: systolic BP ≤ 90 mm Hg or diastolic BP ≤ 60 mm Hg
    • Age 65 years
    • Interpretation
      • Each finding is assigned 1 point.
      • CURB-65 score 0 or 1: The patient may be treated as an outpatient.
      • CURB-65 score ≥ 2: Hospitalization is indicated.
      • CURB-65 score ≥ 3: Consider ICU level of care.
    • CRB-65 score (if serum urea is not known or unavailable)
      • CRB-65 score of 0: The patient may be treated as an outpatient.
      • CRB-65 score of ≥ 1: Hospitalization is recommended.
  • Pneumonia severity index (PSI/PORT score) [18]
    • Patients are assigned to one of five risk classes based on a more complex point system than in CURB-65.
    • Points are distributed based on patient age, comorbidities, and lab results.

The CURB-65 score and PSI are tools for evaluating the risk of mortality. They have not been validated for determining the necessity for ICU admission.

Criteria for ICU admission

IDSA/ATS criteria for severe CAP [10]
Major criteria
Minor criteria


  • Severe CAP: one major criterion or ≥ 3 minor criteria

Empiric antibiotic therapy for community-acquired pneumonia [10][19]


Empiric antibiotic therapy for community-acquired pneumonia in an outpatient setting
Patient profile Recommended empiric antibiotic regimen [10]

Previously healthy patients without comorbidities or risk factors for resistant pathogens

Patients with comorbidities or risk factors for resistant pathogens

  • Duration of treatment
    • 5 days of therapy is usually sufficient for CAP that is treated in the outpatient setting.
    • Any patient being treated in a primary care setting should be re-examined after 48–72 hours to evaluate the efficacy of the prescribed antibiotic.
  • Additional considerations: Knowing local resistance patterns of S. pneumoniae to macrolides is critical when deciding on an empiric antibiotic regimen.


Empiric antibiotic therapy for community-acquired pneumonia in an inpatient setting
Patient profile Recommended empiric antibiotic regimen [10]

Nonsevere CAP/non-ICU treatment

Severe CAP/ICU treatment

Risk factors for Pseudomonas aeruginosa

Risk factors for MRSA

If aztreonam is used as an alternative to other β-lactam antibiotics, additional coverage for MSSA must be included (e.g., a fluoroquinolone).

Empiric antibiotic therapy for hospital-acquired pneumonia [9]

Empiric antibiotic therapy for hospital-acquired pneumonia
Patient profile Recommended empiric antibiotic regimen [9]

Patients not at high risk for mortality and without risk factors for MRSA infection

Patients not at high risk for mortality but with risk factors for MRSA infection

Patients at high risk for mortality

Patients with structural lung disease (e.g., cystic fibrosis, bronchiectasis)

  • Duration of treatment
    • Empiric antibiotic therapy should be narrowed and/or de-escalated as soon as feasible.
    • Seven days of therapy are usually sufficient. [9]
  • Additional considerations: Resistance patterns can vary widely; local antibiograms should be considered when starting empiric treatment.

Patients with structural lung disease and/or at high risk for mortality should receive double antipseudomonal coverage!

Empiric antibiotic therapy for ventilator-associated pneumonia [9]

Supportive therapy

Mycoplasma pneumonia [21]

Other types of pathogen-specific pneumonia



Clinical features




  • Optimize treatment and/or prophylaxis of underlying causes to reduce the risk of aspiration (e.g., elevation of the head of the bed).

We list the most important complications. The selection is not exhaustive.

  • Mortality increases with age.
  • The mortality risk can be evaluated with the CURB-65 score. [25]
    • Score 0: ∼ 1%
    • Score 1–2: ∼ 10%
    • Score 3: ∼ 14%
    • Score 4: ∼ 40%
  • HAP is associated with a mortality rate of > 20%.

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