What blood work is required for a patient with community-acquired pneumonia (CAP)?

Blood Work Required for Community-Acquired Pneumonia

Outpatient Management

For patients with community-acquired pneumonia managed in the outpatient setting, routine blood work is not necessary. 1, 2

• Microbiological investigations are not recommended routinely for outpatients 1
• General laboratory testing provides minimal benefit for disposition or treatment decisions in mild cases 2
• Pulse oximetry should be considered in emergency assessment centers to evaluate oxygenation status 1

Hospitalized Patients - Standard Blood Work

All hospitalized patients require a core panel of blood tests on admission to assess severity and guide management. 1, 2

Essential Laboratory Tests:

• Complete blood count (CBC) with differential to assess leukocytosis and severity 1, 3
• Basic metabolic panel including:
  • Blood urea nitrogen (BUN) 1
  • Serum creatinine 1
  • Electrolytes (sodium, potassium) 1
  • Glucose 1
• Liver function tests (aminotransferases) 1, 3
• Oxygenation assessment via pulse oximetry or arterial blood gas if respiratory/metabolic acidosis suspected 1, 3

These tests serve dual purposes: they contribute to severity scoring systems (such as CURB-65 and Pneumonia PORT) and identify organ dysfunction requiring specific interventions. 1, 2

Microbiological Blood Work

Two sets of blood cultures should be drawn before antibiotic initiation in all hospitalized patients. 1, 3

• Blood cultures have an approximately 11% yield, with Streptococcus pneumoniae being the most commonly identified pathogen 1, 3, 4
• Cultures must be obtained before antibiotics whenever possible, though antibiotic therapy should never be delayed to obtain specimens 3
• Blood cultures help identify bacteremia and resistant pathogens that may require treatment modification 1

Severe CAP - Additional Testing

Patients with severe community-acquired pneumonia requiring ICU admission need expanded testing beyond the standard panel. 2, 3

Mandatory Additional Tests:

• Legionella pneumophila urinary antigen for serogroup 1 detection 1, 2, 3
  • Remains positive for weeks to months after infection, allowing diagnosis even after antibiotic initiation 3
  • Particularly important in severe cases and during outbreaks 1
• Streptococcus pneumoniae urinary antigen testing 1, 2, 3
• Paired serological tests for atypical pathogens in severe cases 1

Special Circumstances

HIV Testing:

• Consider HIV serology with informed consent in patients aged 15-54 years admitted for CAP, especially those with risk factors 1, 3

Pleural Effusion:

• Any significant pleural effusion (≥10mm on lateral decubitus film) requires thoracentesis 1, 3
• Pleural fluid analysis should include: white blood cell count and differential, protein, glucose, LDH, pH, Gram stain, acid-fast stain, and cultures for bacteria, fungi, and mycobacteria 1, 3

Additional Biomarkers (Selected Cases):

• C-reactive protein (CRP) when locally available for hospitalized patients 1
• Procalcitonin may help differentiate bacterial pneumonia from other causes in selected cases 3
• Natriuretic peptides and troponin to differentiate heart failure from pneumonia in patients with dyspnea 2, 3

Common Pitfalls to Avoid

• Do not routinely perform serologic testing or cold agglutinin measurements in the initial evaluation, as they are not useful for acute management 1
• Avoid delaying antibiotic administration to obtain blood cultures or other specimens, as this worsens outcomes 3
• Do not order extensive microbiological workup for every hospitalized patient; tailor testing to severity, epidemiological risk factors, and treatment response 1
• Sputum Gram stain is not routinely necessary for all patients, but should be available for severe CAP or complications 1