What tests should be ordered for a pneumonia workup?

Pneumonia Workup: Testing Strategy Based on Severity and Setting

The testing approach for pneumonia should be stratified by severity and care setting: outpatients with mild pneumonia require minimal to no testing, while hospitalized patients need chest radiography, basic laboratory work, and oxygenation assessment, with microbiological testing reserved for severe cases or specific clinical indications. 1

Outpatient/Community Management

Minimal Testing Approach

• General investigations, including chest radiography, are NOT necessary for the majority of patients with suspected community-acquired pneumonia managed in the community 1
• Routine diagnostic tests to identify an etiologic diagnosis are optional for outpatients 1
• Pulse oximetry should be considered in out-of-hours and emergency assessment centers for simple oxygenation assessment 1

Microbiological Testing in Outpatients

• Microbiological investigations are NOT recommended routinely 1
• Sputum examination should be considered only for patients who fail to respond to empirical antibiotic therapy 1
• Consider sputum for tuberculosis testing in patients with persistent productive cough, especially with malaise, weight loss, night sweats, or TB risk factors (ethnic origin, social deprivation, elderly) 1
• Serological investigations may be considered during outbreaks (e.g., Legionnaires' disease) or epidemic mycoplasma years 1

Hospitalized Patients: Essential Tests on Admission

Mandatory General Investigations

All hospitalized patients should have the following performed on admission: 1

• Chest radiograph 1
• Full blood count (CBC with differential) 1, 2
• Urea, electrolytes, and liver function tests (basic metabolic panel) 1, 2
• C-reactive protein (CRP) when locally available 1 - levels >100 mg/L suggest pneumonia while <20 mg/L make it unlikely 2
• Oxygenation assessment (arterial blood gas or pulse oximetry) 1, 2

Additional Laboratory Studies for Hospitalized Patients

• Arterial blood gas or pulse oximetry determinations 1
• Basic blood chemistry including creatinine, urea nitrogen, aminotransferases, sodium, and potassium 1
• CRP has independent prognostic potential and reflects clinical course 1

Microbiological Testing: Severity-Guided Approach

Blood Cultures

• Blood cultures are recommended for ALL hospitalized patients, preferably before antibiotic treatment is commenced 1
• Should be obtained in all patients with CAP requiring hospitalization 1
• However, recent evidence shows blood cultures have extremely low yield (6.6% positive rate) and limited utility even in severe cases, with appropriate management changes occurring in only 1.8% of cases 3

Sputum Testing

Sputum collection is indicated when: 1

• Non-severe CAP patients who can expectorate purulent samples AND have not received prior antibiotic treatment 1
• Severe CAP or patients who fail to improve 1
• Only if a good-quality specimen can be obtained and quality performance measures for collection, transport, and processing can be met 1

Gram stain considerations: 1

• Should be offered for severe CAP or complications, as it can give immediate indication of likely pathogens 1
• Routine Gram stain on all patients is unnecessary 1
• Gram stain is recommended when a purulent sputum sample can be obtained and is processed timely 1

Severe CAP: Comprehensive Testing Protocol

For patients with severe CAP (defined by ICU admission criteria, respiratory failure, septic shock, or multilobar involvement), obtain: 1, 2

Mandatory Tests

• Blood cultures (at least two sets before antibiotics) 1, 2
• Urinary antigen test for Legionella pneumophila 1, 2
• Urinary antigen test for Streptococcus pneumoniae 1, 2
• Expectorated sputum for culture (or endotracheal aspirate if intubated) 1

Additional Severe CAP Testing

• Paired serological tests for atypical pathogens 1
• Pneumococcal antigen tests if available locally 1
• Legionella culture should be specifically requested for severe CAP 1
• Legionella cultures should be routinely performed on invasive respiratory samples obtained by bronchoscopy 1

Special Circumstances Requiring Extended Testing

Specific Clinical Indications for Microbiological Testing 1

Testing should be performed when results are likely to change individual antibiotic management or have highest yield, including:

• Severe CAP requiring ICU admission 1
• Failure to respond to empirical therapy 1
• Cavitary infiltrates 1
• Leukopenia 1
• Active alcohol abuse 1
• Chronic severe liver disease 1
• Severe obstructive/structural lung disease 1
• Asplenia 1
• Recent travel (within 2 weeks) 1
• Positive Legionella or pneumococcal urinary antigen test 1

Outbreak or Epidemiological Situations

• Serological tests should be extended to all hospitalized patients during outbreaks 1
• Investigations for Legionella infection are recommended for all patients during outbreaks 1
• Rapid testing and reporting for Legionella urine antigen should be available in at least one laboratory per region 1

Unresponsive to β-lactam Antibiotics

• Paired serological tests should be performed 1
• Consider atypical pathogen testing 1

Additional Diagnostic Considerations

Cardiac Evaluation (When Dyspnea Present)

• Natriuretic peptides (BNP or NT-proBNP) to differentiate heart failure from pneumonia - low levels make left ventricular failure unlikely 2
• Troponin to rule out myocardial injury, especially with coronary artery disease risk factors 2
• ECG to assess for arrhythmias or ischemic changes 2

Other Specialized Tests

• D-dimer if pulmonary embolism is in the differential diagnosis 2
• Procalcitonin (PCT) to distinguish bacterial from viral pneumonia and guide antibiotic therapy decisions 2
• Diagnostic thoracentesis when significant pleural effusion is present 1

Invasive Procedures (Selected Cases Only)

• Trans-thoracic needle aspiration can only be considered on an individual basis for severely ill patients with focal infiltrate when less invasive measures are nondiagnostic 1
• Bronchoscopic protected specimen brush (PSB) and bronchoalveolar lavage (BAL) can be considered in intubated patients and selected nonintubated patients where gas exchange allows 1

Key Clinical Pitfalls to Avoid

• Do not order extensive microbiological workup for mild outpatient pneumonia - it is unnecessary and not cost-effective 1
• Do not rely solely on chest radiograph - sensitivity is only 46-77%, and misdiagnosis occurs in up to one-third of patients 4
• Do not delay antibiotics waiting for culture results - blood cultures should be obtained before antibiotics but should not delay treatment 1
• Do not perform routine blood cultures in all pneumonia cases - recent evidence shows extremely low yield (0.4% multidrug-resistant organisms) and minimal impact on management 3
• Ensure quality sputum specimens - contamination with oropharyngeal flora is common, and approximately 40% of patients cannot expectorate 5