Is a Blood Test Needed to Detect or Prove Influenza Infection?
No, blood tests (serology) are not recommended for detecting acute influenza infection because they cannot provide timely results that influence clinical management. 1
Why Blood Tests Are Not Useful for Acute Diagnosis
Serological testing requires paired serum samples from both acute and convalescent phases, making results unavailable when treatment decisions need to be made. 1 Single acute-phase serum samples cannot be reliably interpreted for influenza diagnosis. 1 The antibody tests (hemagglutination inhibition, ELISA, or complement fixation) are only available through reference laboratories and are useful solely for retrospective diagnosis, surveillance, or research—not for guiding acute clinical care. 1
Recommended Diagnostic Approaches Instead
Respiratory Specimen Testing (Not Blood)
The diagnosis of influenza requires respiratory tract specimens, not blood samples. 1 The preferred diagnostic tests, in order of priority, are:
RT-PCR (reverse transcription polymerase chain reaction): This is the most sensitive and specific test, with results available in 2-6 hours, and is highly recommended as the gold standard. 1
Immunofluorescence testing: Direct or indirect fluorescent antibody staining provides moderately high sensitivity and high specificity, with results in 2-4 hours. 1
Rapid antigen detection tests: These have moderate sensitivity (20-70% in adults, 70-90% in children) but high specificity (>90%), with results in 10-30 minutes. 1, 2
Optimal Specimen Collection
Nasopharyngeal specimens should be collected as the first choice to maximize viral detection. 1 If unavailable, combined nasal and throat swabs are preferred over single throat swabs. 1 Specimens should be collected within 4-5 days of symptom onset when viral shedding is highest in immunocompetent patients. 1, 3
Special Populations Requiring Different Approaches
High-Risk Patients
For elderly patients, young children, and those with underlying conditions, testing should be performed if results will influence clinical management decisions, particularly regarding antiviral treatment initiation. 1 In immunocompromised patients, viral shedding can persist for weeks to months, so specimens may remain positive well beyond the typical 5-day window. 1, 3
When Clinical Diagnosis May Suffice
For otherwise healthy outpatients presenting during peak influenza season with typical symptoms (fever, cough, myalgias, acute onset), laboratory confirmation may not be necessary if the diagnosis is clinically apparent and will not change management. 4 However, the positive predictive value of clinical diagnosis alone is limited—only 30% in patients ≥60 years and 53% in hospitalized elderly patients with chronic conditions. 2
Complete Blood Count Considerations
While a complete blood count (CBC) is not diagnostic for influenza, it may be indicated for hospitalized patients to assess severity and detect complications. 5 Leukocytosis with left shift may suggest bacterial superinfection, while lymphopenia has been noted in severe viral infections. 5 However, the CBC does not confirm or exclude influenza infection itself.
Common Pitfalls to Avoid
Do not order serology for acute diagnosis—results will not be available in time to guide treatment, which must be initiated within 48 hours of symptom onset for maximum benefit. 1, 4
Do not assume negative rapid tests rule out influenza, especially after 5 days of symptoms or in adults where sensitivity is poor (40-60%). 1, 3 Consider confirmatory RT-PCR if clinical suspicion remains high. 3
Do not delay specimen collection—collect as soon as possible after illness onset and refrigerate (do not freeze) until analysis. 1, 3
Do not rely solely on throat swabs—these have lower detection rates than nasopharyngeal specimens. 1, 3