Evaluation of Rapid Influenza Diagnostic Tests

The August 6, 2009, Morbidity and Mortality Weekly Report article,"Evaluation of Rapid Influenza Diagnostic Tests for Detection of Novel Influenza A (H1N1) Virus—United States, 2009," reported on an evaluation of the ability of three commercially available rapid influenza diagnostic tests (RIDTs) to detect novel influenza A (H1N1). RIDTs from three companies were reviewed and results indicate that these tests can detect novel influenza A (H1N1) in respiratory specimens, but the overall sensitivities ranged from 40–69 percent compared to a real time polymerase chain reaction (rt-PCR) test. This means that many influenza infections will be missed by RIDTs.

Given the lower sensitivities found with RIDTs, decisions regarding treatment and further testing among patients with negative results from RIDT testing should be based upon clinician suspicion, underlying medical conditions, severity of illness and risk for complications in those persons suspected of having novel H1N1 virus infection.

Points to Consider:

• Rapid Influenza Diagnostic tests (RIDTs) are tests that detect influenza A or B antigens and can provide results within 15 minutes.

• The study found that commonly used RIDTs are capable of detecting novel influenza A (H1N1) from respiratory samples containing high virus titers, but sensitivities declined substantially as cycle threshold (Ct*) values increase.

* Cycle threshold (Ct) values are indicators of the amount of virus in a sample. Lower cycle threshold values indicate higher amounts of viral material in the specimen.

• All RIDTs performed well compared to rRT-PCR for samples with cycle threshold (Ct)* values less than 20 with 89–100 percent sensitivity.
• Sensitivities of the RIDTs was highest among specimens with Ct values of less than 20.
• Among samples with Ct values of 20 or greater, the sensitivity declined substantially.
• Given the lower sensitivity of RIDTs compared to other assays, such as rRT-PCR, clinicians should understand that a negative result by RIDT does not exclude influenza virus infection.
• The findings from this study are similar to other recent studies, which reported that the sensitivity of some RIDTs to detect novel influenza A (H1N1) in clinical specimens ranged from 10–51 percent compared with rRT-PCR.
• CDC's guidance on interpretation of RIDTs for testing of patients with suspected novel H1n1 virus infection is available on CDC's H1N1 Flu page.