Bloodwork for Influenza Diagnosis
Blood tests should NOT be routinely ordered for diagnosing influenza—respiratory specimen testing (not bloodwork) is the appropriate diagnostic approach. 1
Key Diagnostic Principle
The IDSA 2019 guidelines explicitly state that clinicians should not collect or routinely test specimens for influenza from nonrespiratory sites such as blood, plasma, serum, cerebrospinal fluid, urine, and stool 1. This is a strong recommendation (A-III level evidence).
What Should Be Done Instead
Respiratory specimens are the gold standard for influenza diagnosis:
- Outpatients: Nasopharyngeal swabs (preferred), or mid-turbinate nasal swabs, or combined nasal-throat swabs 1
- Hospitalized patients: Same respiratory specimens as outpatients for those without severe respiratory failure 1
- Mechanically ventilated patients: Endotracheal aspirate or bronchoalveolar lavage fluid 1
Testing method by setting:
- Outpatients: Rapid molecular assays (nucleic acid amplification tests) or rapid influenza diagnostic tests (RIDTs) 1
- Hospitalized patients: RT-PCR or other molecular assays (NOT RIDTs unless molecular assays unavailable) 1
Why Serology/Blood Tests Are Not Recommended
Serologic testing should not be used for influenza diagnosis because:
- Single serum specimens cannot be reliably interpreted 1
- Paired acute/convalescent sera collected 2-3 weeks apart would be needed, which is not clinically useful for acute management 1
Role of Routine Blood Counts (CBC)
While not diagnostic for influenza, routine complete blood counts may have supportive clinical value for:
Prognostic Assessment
Recent research suggests certain hematological parameters correlate with influenza severity:
- Neutrophil-to-lymphocyte ratio (NLR): Elevated in influenza B patients, particularly those requiring hospitalization (threshold >2.36 with 96.7% sensitivity) 2
- Platelet counts: Significantly lower in influenza virus infection compared to other respiratory infections 3
- Lymphocyte × Platelet (LYM*PLT): Shows screening potential in pediatric populations 4
Clinical Context
These parameters may help:
- Differentiate influenza from bacterial infections (NLR, platelet-to-neutrophil ratio) 3
- Assess disease severity and need for hospitalization 2
- Monitor treatment response (platelet recovery correlates with clinical improvement) 3
Critical Caveat
Do not delay antiviral treatment while awaiting any test results. For patients meeting high-risk criteria (hospitalized, severe illness, immunocompromised, age <2 or ≥65 years, pregnant), start oseltamivir immediately based on clinical suspicion 1. Testing confirms diagnosis but should not delay treatment initiation.