Pneumonia%20-%20community-acquired Diagnosis

• 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 with 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, asplenia], if they may be treated as outpatients with a reasonable assurance for follow-up and sufficient social support
• 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
  • Without 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
• Possible pathogens: S pneumoniae, H influenzae, Chlamydophila pneumoniae, M pneumoniae, M catarrhalis, enteric Gram-negative bacilli, L pneumophila and anaerobes (in patients with risk of aspiration)
• 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 eg endocarditis, arthritis, encephalitis, otitis media
  • 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%
• Possible pathogens: S pneumoniae, H influenzae, Chlamydophila pneumoniae, M pneumoniae, M catarrhalis, enteric Gram-negative bacilli, L pneumophila, S aureus, P aeruginosa, Pneumocystis jiroveci, Klebsiella pneumoniae, Acinetobacter and anaerobes (in patients with risk of aspiration)
• Patients classified as severe CAP should be managed in a hospital setting and those with hemodynamic instability or respiratory failure should be admitted in the ICU
• Clinical Features:
  • Patient exhibits either 1 major criteria or ≥3 minor criteria:
    • Minor criteria: Respiratory rate ≥30 breaths/minute, PaO₂/FiO₂ ratio of ≤250 mmHg, multilobar infiltrates and confusion/disorientation, blood urea nitrogen ≥20 mg/dL, WBC <4000 cells/μL, platelet count <100,000/μL, core temperature <36°C, hypotension needing aggressive fluid resuscitation
    • Major criteria: Septic shock needing vasopressors, respiratory failure needing 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

• Clinical judgment with a validated clinical prediction rule should be used to determine the best site of care for patients
  • Other factors to be considered in determining site of care aside from clinical severity include ability of patient to maintain oral intake, cognitive or functional impairment, likelihood of medication adherence, history of substance abuse, severe comorbid illness, mental illness, and living or social circumstances
• Patients with severe CAP should be managed in a hospital setting
• Direct ICU admission is recommended for patients with hypotension or shock requiring vasopressors or respiratory failure needing mechanical ventilation

CRB65/CURB65 Score for Mortality Risk Assessment

• May be used as a prognostic baseline for patients with community-acquired pneumonia (CAP)
• CRB65 is used in primary care; CURB65 is used for hospitalized patients

Prognostic Features (1 point for each feature present)

• Confusion (abbreviated Mental Test score <8 or new disorientation)
• CURB65: Increased blood urea nitrogen (>7 mmol/L or >20 mg/dL) (*CURB65; for hospitalized patients only)
• Increased respiratory rate (RR) [≥30 breaths/minute]
• Decreased blood pressure (BP) [<90/≤60 mmHg]
• ≥65 years old

Stratification:

• CRB65: 
  • 0: Low-risk (<1% mortality risk)
  • 1-2: Intermediate-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, and 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)
• CURB65
  • 0-1: Low-risk (<3% mortality risk)
  • 2: Intermediate-risk (3-15% mortality risk)
  • 3-5: High-risk (>15% mortality risk)

Pneumonia Severity Index (PSI)

• Adopted and preferred by the American Thoracic Society, used to classify patients accordingly to mortality risk and predicts the need for hospitalization
• Parameters include (step 1) age, comorbidities, physical exam, and (step 2) gender, laboratory and imaging findings:
  • Demographic factors:
    • Men: Age (yr)
    • Women: Age (yr) - 10
  • Nursing home residents: + 10
  • Comorbid illnesses:
    • Neoplastic disease: + 30
    • Liver disease: + 20
    • Congestive heart failure: + 10
    • Cerebrovascular disease: + 10
    • Renal disease: + 10
  • Physical examination findings:
    • Altered mental status: + 20
    • Respiratory rate ≥30 breaths/min: + 20
    • Systolic blood pressure <90 mmHg: + 20
    • Temperature <35^oC or ≥40^oC: + 15
    • Pulse ≥125 beats/min: + 10
  • Laboratory and imaging findings:
    • Arterial pH <7.35: + 30
    • BUN ≥30 mg/dL (≥10.7 mmol/L): + 20
    • Sodium <130 mEq/L: + 20
    • Glucose ≥250 mg/dL (≥13.9 mmol/L): + 10
    • Hematocrit <30% PaO₂ <60 mmHg or O₂ saturation <90%: + 10
    • Pleural effusion: + 10
• A risk score is obtained by adding the points for each applicable patient characteristic

Risk Class

• I (Low-risk): When no predictors are present
• II (Low-risk): ≤70 points; mortality estimated at 0.6%
  • Outpatient therapy except for patients with respiratory rate ≥30 breaths/minute, PaO₂/FiO₂ ratio of ≤250 mmHg, systolic BP <90 mmHg, diastolic BP <60 mmHg, multilobar infiltrates and confusion
• III (Low-risk): 71-90 points; mortality estimated at 0.9%
  • May be managed as outpatient depending on clinical status, available resources and other factors
  • Option for at-home parenteral antibiotic therapy or brief hospitalization (<24 hours)
• IV (Moderate-risk): 91-130 points; mortality estimated at 9.3%; inpatient therapy
• V (High-risk): >130 points; mortality estimated at 27%; inpatient therapy

Other Diagnostic Tests

• 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; >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
• C-reactive protein (CRP) and procalcitonin may be obtained for diagnosis and assessment of response to therapy
  • Procalcitonin may be used to guide the initiation and duration of antibiotic treatment
    • Non critically-ill patients with procalcitonin level of <0.25 μg/L rules out bacterial infection and may not benefit from antibiotic therapy
    • Critically-ill patients with procalcitonin level of <0.50 g/L indicates a low probability of bacterial infection; however, retesting after 6-24 hours is needed to re-evaluate the need for empiric antibiotic therapy

Microbial Etiology Testing
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, SARS coronavirus 2 (SARS-CoV-2/COVID-19)]
• 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 with low-risk community-acquired pneumonia (CAP)

Blood Culture

• Although of low sensitivity, a positive blood culture is specific and is considered the gold standard in the etiologic diagnosis of pneumonia
• Pretreatment blood culture is recommended for patients with severe CAP, currently taking empiric agents for methicillin-resistant S aureus (MRSA) or P aeruginosa, previously infected with MRSA or P aeruginosa especially those with prior lung infection, or with history of hospitalization given parenteral antibiotics during admission or within the last 90 days
• Get at least 2 sets of pretreatment blood cultures
• 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

• Pretreatment Gram stain and blood culture of lower respiratory tract aspirate is recommended for patients with severe CAP especially if intubated, currently being treated for MRSA or P aeruginosa, with history of MRSA or P aeruginosa infection especially those with prior lung infection, or with history of hospitalization given parenteral antibiotics during admission or within the last 90 days 
• For intubated patients, an endotracheal aspirate sample or a brochoalveolar lavage from bronchoscopy 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

Serology

• Urine antigen test or nucleic acid amplification test for Legionella sp should be done if epidemiologically relevant (eg Legionella outbreak), recent travel, and in adults with severe CAP 
• May perform additional serological 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

Rapid Molecular Diagnostic Techniques

• Rapid molecular tests (eg PCR, DNA microarray, mass TagPCR, Binax Now S pneumoniae test) yield results faster, usually in 1-2 hours, compared to conventional diagnostics
• PCR is based on DNA detection of pathogens in respiratory secretions, nasopharyngeal swabs and blood, with high specificity and sensitivity for S pneumoniae and L pneumoniae
• With faster results, high sensitivity and moderate-high specificity for M pneumoniae but may be affected by certain concerns in availability, cost and technical issues
• May be used as a guide in escalation and de-escalation of antibiotic therapy
• Preferred over rapid diagnostic tests (eg antigen test) for the detection of influenza
  • PCR is the gold standard due to its high specificity and sensitivity, greater time window for detection, and faster results

Chest Radiography (Chest X-ray) 

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