How is pneumonia classified?

Classification of Pneumonia

Pneumonia is primarily classified by setting of acquisition (community vs. healthcare-associated vs. hospital-acquired), severity of illness, and need for hospitalization, with the setting-based classification being most critical for guiding appropriate empirical antibiotic therapy. 1, 2

Classification by Setting of Acquisition

Community-Acquired Pneumonia (CAP)

• CAP is defined as pulmonary parenchymal infection acquired in the community, not in a healthcare setting. 2
• This represents the traditional classification for pneumonia developing outside the hospital in patients without recent healthcare exposure. 3

Healthcare-Associated Pneumonia (HCAP) - Now Abandoned

• The 2019 ATS/IDSA guidelines recommend abandoning the HCAP category entirely, as the positive predictive value for multidrug-resistant (MDR) bacteria was far too low to justify routine broad-spectrum empirical coverage. 1
• Instead of using HCAP as a category, emphasis should be placed on local epidemiology and validated individual risk factors to determine need for MRSA or Pseudomonas aeruginosa coverage. 1
• Risk factors for MDR organisms include: antimicrobial therapy in preceding 90 days, current hospitalization ≥5 days, high frequency of antibiotic resistance in the community, hospitalization for ≥2 days in preceding 90 days, residence in nursing home, home infusion therapy, chronic dialysis within 30 days, home wound care, family member with MDR pathogen, and immunosuppressive disease/therapy. 2

Hospital-Acquired Pneumonia (HAP) and Ventilator-Associated Pneumonia (VAP)

• HAP develops after at least 48 hours of hospital admission and is bacteriologically similar to healthcare-associated infections. 1
• Hospitalization for at least 5 days increases the risk of infection with MDR organisms. 1

Classification by Severity

Severity Assessment Tools

• CURB-65 and Pneumonia Severity Index (PSI) are the most recommended systems for assessing severity and guiding site-of-care decisions. 4
• CURB-65 evaluates: Confusion, Urea >7 mmol/L, Respiratory rate ≥30/min, Blood pressure (systolic <90 mmHg or diastolic ≤60 mmHg), age ≥65 years. 4
• PSI stratifies patients into 5 risk classes based on age, comorbidities, and clinical parameters. 4

Site-of-Care Decision

• Patients with CURB-65 score 0-1 or PSI classes I-II are candidates for outpatient treatment. 4
• This decision must be integrated with assessment of ability to take oral medications safely and availability of home support. 4
• Patients with CURB-65 score ≥2 require hospital admission. 4
• The site-of-care decision is the single most important clinical decision, directly affecting testing intensity, antibiotic therapy, and costs (average inpatient cost $7,500 vs. outpatient $150-$350). 1

Severe CAP Requiring ICU Admission

• Patients with persistent septic shock despite adequate fluid resuscitation or requiring vasopressor support should be admitted to ICU. 1
• Patients with severe hypoxemia (PaO₂/FiO₂ ratio <150) and bilateral alveolar infiltrates require immediate intubation. 1

Classification by Etiology - Limited Clinical Utility

Typical vs. Atypical Pneumonia - Not Reliable

• The classification of pneumonia into "typical" and "atypical" forms has limited clinical value and cannot be used to reliably establish etiologic diagnosis. 1
• Clinical features, including history, physical examination, and routine laboratory and roentgenographic evaluation, cannot reliably make a specific etiologic diagnosis of CAP. 1
• No roentgenographic pattern is sufficiently distinctive to allow classification of individual cases. 1
• Host factors (advanced age, coexisting illness) are often just as important as the pathogen identity in defining presenting signs and symptoms. 1

Diagnostic Approach

• Diagnosis requires a constellation of clinical features suggestive of pneumonia plus an infiltrate demonstrated by chest radiography or other imaging, with or without microbiological data. 4
• Clinical signs and symptoms cannot be used reliably to establish etiologic diagnosis with adequate sensitivity and specificity. 4

Common Pitfalls to Avoid

• Do not rely on a single severity score without incorporating clinical judgment, as this can lead to inappropriate site-of-care decisions. 4
• Do not fail to reassess severity regularly during the disease course, as this prevents appropriate management adjustments. 4
• Do not use the HCAP classification to automatically prescribe broad-spectrum antibiotics, as this leads to overtreatment; instead, use validated individual risk factors for MDR organisms. 1
• Do not delay appropriate antimicrobial therapy for diagnostic studies in clinically unstable patients, as delays increase mortality. 1