What laboratory tests are recommended for a patient with influenza A?

Laboratory Testing for Influenza A Positive Patients

For patients with confirmed influenza A, routine laboratory tests beyond the diagnostic virology are generally not necessary in outpatient settings, but hospitalized patients should have basic blood work to assess severity and guide management. 1

Outpatient Setting

No routine laboratory tests are recommended for uncomplicated influenza A in otherwise healthy outpatients. 2 Once influenza is confirmed, the focus shifts to clinical management and antiviral treatment rather than additional laboratory investigation.

• The diagnosis has already been established virologically, and further testing does not change management in uncomplicated cases 1
• Treatment decisions should be based on clinical presentation and risk factors, not laboratory parameters 1

Hospitalized Patients

For patients admitted to hospital with influenza A, obtain the following baseline laboratory tests: 1

Essential Blood Tests

• Complete blood count (CBC): May reveal leukocytosis with left shift in bacterial coinfection or lymphopenia in severe viral disease 1
• Urea, creatinine, and electrolytes: To detect renal impairment or electrolyte disturbances (hypo- or hypernatremia) 1
• Liver function tests: Usually normal but should be checked as baseline 1
• Creatine kinase: If myositis is suspected based on severe myalgias 1

When to Investigate for Bacterial Coinfection

Empirically investigate and treat bacterial coinfection in the following scenarios: 1

• Patients presenting initially with severe disease (extensive pneumonia, respiratory failure, hypotension, persistent fever) - obtain blood cultures, sputum cultures, and consider urinary antigen testing for S. pneumoniae 1
• Patients who deteriorate after initial improvement, particularly those already on antivirals 1
• Patients who fail to improve after 3-5 days of antiviral treatment 1

The most likely bacterial pathogens are Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, and beta-hemolytic streptococci 1

C-Reactive Protein (CRP)

• CRP may aid diagnosis when bacterial superinfection is suspected, though its diagnostic value in lower respiratory tract infections remains controversial 1
• CRP is not routinely helpful for uncomplicated influenza 1

Tests NOT Recommended

Do not routinely collect or test the following: 1

• Non-respiratory specimens (blood, plasma, serum, cerebrospinal fluid, urine, stool) for influenza testing 1
• Serological testing (single or paired sera) for clinical management purposes - results are not timely and require paired specimens 2-3 weeks apart 1

Special Considerations for Antiviral Resistance Testing

Consider neuraminidase inhibitor (NAI) resistance testing in specific circumstances: 1

• Patients who develop laboratory-confirmed influenza while on or immediately after NAI chemoprophylaxis 1
• Immunocompromised patients with persistent viral replication (after 7-10 days) who remain ill during or after NAI treatment 1
• Patients who inadvertently received subtherapeutic NAI dosing 1
• Patients with severe influenza who fail to improve with NAI treatment and have evidence of persistent viral replication 1

Critical Clinical Pitfalls

• Do not delay antiviral treatment while waiting for any laboratory results - treatment efficacy is greatest when started within 24 hours of symptom onset 1, 3
• Do not assume absence of bacterial coinfection based on initial presentation alone - secondary bacterial pneumonia is a severe complication that requires prompt recognition 1
• Do not use laboratory parameters to decide whether to treat high-risk patients - clinical judgment and risk stratification should guide antiviral therapy decisions 1