From the Guidelines
A patient will typically test positive for influenza for up to 7 days after illness onset, as reported in the context of live attenuated influenza vaccine (LAIV) receipt. The duration of a positive test depends on several factors, including:
- Test type: Different tests have varying sensitivities and specificities, which can affect the duration of positive results, as noted in the 2008 advisory committee on immunization practices (ACIP) recommendations 1.
- Specimen quality and timing: The quality of the specimen and the timing of collection in relation to illness onset can impact test results, with nasopharyngeal and nasal specimens generally having higher yields than throat swab specimens 1.
- Individual patient factors: Immune status, age, and other individual factors can influence the duration of viral shedding and positive test results. It's essential to consider these factors and the specific test used when interpreting influenza test results, as the sensitivity and specificity of tests can vary by laboratory, test type, and specimen quality 1.
From the Research
Duration of Influenza A Virus Shedding
- The duration of influenza A virus shedding can vary, with some studies suggesting that patients can shed detectable virus beyond the 5- to 7-day period traditionally considered the duration of infectivity 2.
- A study found that 54% of patients had influenza A virus detected by PCR and 29% by culture methods at or beyond 7 days after symptom onset 2.
- Another study found that the clearance rate of influenza virus antigen was 9.8% within 5 days after onset, 32.1% within 5-7 days, and 98.1% within 7-10 days after onset 3.
Factors Affecting Duration of Shedding
- The duration of shedding may be affected by factors such as age, underlying chronic medical conditions, and receipt of antiviral treatment 2.
- A study found that oseltamivir treatment significantly reduced virus isolation on days 2,4, and 7 in patients with influenza 4.
Implications for Infection Control
- The findings of these studies have implications for infection control, suggesting that hospitalized patients with influenza A virus infection may need to be isolated for longer than the traditional 5- to 7-day period to prevent nosocomial outbreaks 2.
- The use of point-of-care tests for influenza can help reduce the time to isolation and improve infection control 5.