Role of TLC and DLC in Managing Influenza Fever
Full blood count (TLC and DLC) should be performed in all patients hospitalized with influenza fever as part of the standard diagnostic workup, with particular attention to lymphopenia which may indicate severe viral infection. 1, 2
Diagnostic Value of Blood Count in Influenza
- Full blood count is recommended as a standard investigation for all patients admitted to hospital with influenza-like illness or suspected influenza 1
- Leukocytosis with left shift (increased neutrophils) may indicate primary viral pneumonia, mixed viral-bacterial pneumonia, or secondary bacterial pneumonia 1, 2
- Lymphopenia (decreased lymphocyte count) is a common finding in influenza infection, observed in up to 96.4% of patients, and may indicate severe disease, particularly in avian H5N1 influenza 2, 3
- Eosinopenia (decreased eosinophil count) is seen in approximately 50% of influenza cases 3
- White blood cell counts above 8000/mm³ are associated with a lower probability of influenza and may suggest bacterial infection 4
Clinical Utility in Disease Monitoring
- Serial monitoring of TLC and DLC can help track disease progression and response to treatment 2
- The granulocyte to lymphocyte ratio in blood represents an early indicator of severe disease progression, detectable as early as two days after influenza infection 5
- Influenza B infections tend to have lower leukocyte counts (6383 ± 3970/mm³) compared to influenza A infections (7639 ± 3476/mm³) 6
- A negative correlation exists between IFN-γ and IP-10 levels with lymphocyte count in seasonal influenza infection 7
Additional Laboratory Tests to Consider
- When obtaining TLC and DLC, the following additional tests should also be performed 1:
- Urea and electrolytes to assess for hypo/hypernatremia and renal impairment
- Liver function tests to monitor for hepatic involvement
- C-reactive protein (CRP) if secondary bacterial infection is suspected
- Creatine kinase if myositis is suspected
Testing Strategy Based on Pandemic Phase
- During early pandemic phases (UK alert levels 1,2, and 3), comprehensive testing including TLC and DLC should be performed on all hospitalized cases 1, 2
- In later pandemic phases with higher caseloads (UK alert level 4), testing may be limited to severe cases or those not responding to empirical therapy 1, 2
- For non-severe pneumonia (CURB-65 Score 0,1, or 2), routine blood testing may still be indicated but microbiological testing can be limited 1
- For severe pneumonia (CURB-65 Score 3,4, or 5, or bilateral CXR changes), comprehensive testing including blood cultures should be performed 1
Interpretation and Clinical Decision Making
- Lymphopenia with normal or elevated neutrophils suggests viral infection 2, 3
- Leukocytosis with neutrophilia suggests bacterial superinfection and may indicate need for antibiotics 2
- Rising WBC count during the course of illness may predict increasing severity 3
- Normal WBC count with lymphopenia is common in uncomplicated influenza 3, 6
Common Pitfalls to Avoid
- Do not rely solely on TLC and DLC for diagnosis of influenza, as these findings are not specific 2, 4
- Do not delay antiviral treatment while waiting for laboratory results in high-risk patients 8
- Remember that normal WBC count does not exclude influenza infection 4, 6
- Do not overlook the possibility of bacterial co-infection even with typical viral patterns on blood count 2