When is a complete blood count (CBC) recommended in patients with viral respiratory infections?

Complete Blood Count (CBC) in Viral Respiratory Infections

A complete blood count (CBC) is recommended for patients with viral respiratory infections who are admitted to the hospital or have severe illness, but is not routinely needed for uncomplicated cases managed in outpatient settings. 1

Indications for CBC Testing in Viral Respiratory Infections

Recommended for:

• Patients requiring hospitalization due to severity of illness 1

  • The British Infection Society and British Thoracic Society specifically recommend full blood count for all patients admitted to hospital with influenza-like illness 1

• Patients with suspected complications of viral respiratory infections 2, 3

  • Primary viral pneumonia
  • Secondary bacterial pneumonia
  • Mixed viral-bacterial pneumonia

• Patients with severe illness as indicated by:

  • High CURB-65 score (3 or more) 1
  • Respiratory distress (respiratory rate >30/min) 1
  • Hypoxemia (oxygen saturation <92%) 1
  • Hemodynamic instability 1

• Immunocompromised patients with respiratory symptoms 2, 4

Not Routinely Recommended for:

• Uncomplicated viral respiratory infections managed in outpatient settings 5
• Mild cases with no risk factors for complications 5

Clinical Value of CBC in Viral Respiratory Infections

Diagnostic Value:

• Leukocytosis with left shift may indicate:

  • Primary viral pneumonia
  • Mixed viral-bacterial pneumonia
  • Secondary bacterial pneumonia 1

• Lymphopenia has been noted in severe viral infections, particularly avian H5N1 influenza 1

• Limitations:

  • CBC lacks specificity to reliably distinguish between bacterial and viral infections 5
  • WBC counts do not consistently correlate with disease severity 5
  • False negatives can occur, especially early in disease course 5

Additional Laboratory Tests to Consider

When CBC is indicated, consider also obtaining:

• Urea and electrolytes to assess for hypo/hypernatremia and renal impairment 1, 3
• Liver function tests to monitor for hepatic involvement 1
• Creatine kinase if myositis is suspected 1
• C-reactive protein (CRP) when secondary bacterial infection is suspected 1

Special Considerations

Bacterial Superinfection:

• Consider additional testing when bacterial superinfection is suspected:
  • Blood cultures (preferably before antibiotic administration) 1
  • Sputum gram stain and culture 1
  • Pneumococcal and Legionella urinary antigen tests 1, 3

Pandemic Situations:

• During influenza pandemics, testing strategies may change based on phase and caseload 1
• In early pandemic phases, more comprehensive testing is recommended 1
• In later phases with higher caseloads, testing may be limited to severe cases 1

Common Pitfalls to Avoid

• Overreliance on CBC for diagnosis - CBC alone cannot reliably distinguish viral from bacterial infections 5
• Delaying treatment while waiting for test results - The Infectious Diseases Society of America advises against waiting for test results before initiating treatment in high-risk patients 2
• Failing to consider clinical context - CBC results should be interpreted in conjunction with clinical presentation, imaging findings, and other laboratory tests 3, 5

Algorithm for CBC Testing in Viral Respiratory Infections

1. Outpatient with mild symptoms, no risk factors:

   • CBC not routinely indicated 5

2. Outpatient with risk factors or moderate symptoms:

   • Consider CBC if clinical management would be influenced 2, 3

3. Patient requiring hospitalization:

   • CBC recommended 1
   • Also obtain urea, electrolytes, liver function tests 1

4. Severe illness or suspected complications:

   • CBC essential 1
   • Add additional tests including blood cultures, sputum cultures, and viral testing 1, 3