What is the recommended approach for influenza testing and treatment in patients presenting with influenza-like symptoms?

Recommended Approach for Influenza Testing and Treatment

Influenza testing should be performed in high-risk patients with influenza-like symptoms, with nasopharyngeal specimens collected within 4 days of symptom onset, and antiviral treatment should be initiated promptly without waiting for test results in those at high risk of complications. 1

Testing Recommendations Based on Clinical Setting

Outpatient Setting

• Test for influenza in high-risk patients (including immunocompromised persons) who present with influenza-like illness, pneumonia, or nonspecific respiratory illness if testing will influence clinical management 1
• Test patients with acute respiratory symptoms with/without fever who have exacerbation of chronic medical conditions (asthma, COPD, heart failure) or known complications of influenza 1
• Consider testing for patients not at high risk if results might influence treatment decisions or reduce unnecessary antibiotics 1
• Collect upper respiratory tract specimens as soon as possible after illness onset, preferably within 4 days of symptom onset 1
• Nasopharyngeal specimens are preferred over other upper respiratory tract specimens to increase detection of influenza viruses 1

Hospital Setting

• Test all patients requiring hospitalization with acute respiratory illness, including pneumonia, with or without fever during influenza activity 1
• Test all hospitalized patients with acute worsening of chronic cardiopulmonary disease 1
• Test all immunocompromised patients or those at high risk of complications who present with acute respiratory symptoms 1
• Test all hospitalized patients who develop acute respiratory symptoms without a clear alternative diagnosis 1

Diagnostic Test Selection

• Rapid molecular assays are preferred for influenza diagnosis due to their high accuracy and fast results 2
• Rapid antigen tests have lower sensitivity (10-80%) compared to molecular methods (86-100%) 1
• Negative results from rapid antigen tests should not be used to rule out influenza due to potential false-negatives, especially when clinical suspicion is high 1
• PCR confirmation should be considered in hospitalized patients 1
• Rapid antigen tests are most sensitive when performed within the first 48 hours of symptom onset 3

Treatment Recommendations

• Antiviral treatment should be started as soon as possible after illness onset and should not be delayed while waiting for test results 1
• Treatment provides greatest benefit when started within 24 hours of symptom onset 2
• Oseltamivir is indicated for treatment of acute, uncomplicated influenza in patients 2 weeks of age and older who have been symptomatic for no more than 48 hours 4
• For non-severe influenza, baloxavir is conditionally recommended if the risk of severe illness is high 5
• For severe influenza, oseltamivir is conditionally recommended 5
• Antibiotics should not be used if bacterial co-infection is unlikely 5

Special Considerations

Timing of Testing and Treatment

• Late diagnosis of influenza (4-7 days after symptom onset) is associated with inappropriate antiviral therapy and more complicated presenting features 6
• Early diagnosis and treatment significantly reduce the risk of complications, especially in older adults 6

Common Pitfalls to Avoid

• Relying solely on clinical diagnosis, which has low sensitivity (36%) for detecting influenza 7
• Waiting for test results before initiating treatment in high-risk patients 1
• Testing too early or too late in the course of illness, which can lead to false negative results 3
• Using inappropriate specimen collection techniques that may reduce test sensitivity 3

Additional Investigations for Hospitalized Patients

• Full blood count, urea and electrolytes, liver function tests, and chest X-ray should be performed in all hospitalized patients 1
• Pulse oximetry should be performed in all patients, with arterial blood gases if oxygen saturation is <92% on air 1
• For patients with suspected influenza-related pneumonia, consider blood cultures, pneumococcal and Legionella urine antigen tests, and sputum cultures 1

By following these evidence-based recommendations for influenza testing and treatment, clinicians can improve patient outcomes and reduce unnecessary antibiotic use and hospitalization.