By Carol A. Kemper, MD, FACP

Clinical Associate Professor of Medicine, Stanford University, Division of Infectious Diseases, Santa Clara Valley Medical Center

Dr. Kemper reports no financial relationships relevant to this field of study.

SOURCES: California Department of Public Health. Testing and treatment for patients hospitalized with suspected influenza. Oct. 2, 2017.

Merckx J, Wali R, Schiller I, et al. Diagnostic accuracy of novel and traditional rapid tests for influenza infection compared with reverse transcriptase polymerase chain reaction: A systematic review and meta-analysis. Ann Intern Med 2017;167:394-409.

As we approach flu season, I like to remind providers that rapid influenza diagnostic tests are imperfect — and before ordering a rapid flu test, consider the likelihood of a positive result. Do they plan to treat the patient or the test result?

Merckx et al performed a meta-analysis of more than 162 diagnostic studies, comparing the diagnostic accuracy and sensitivity of rapid influenza diagnostic tests (RIDTs) with immunoassays (DIAs) and nucleic acid amplification tests (NAATs) in children and adults with influenza-like illness (ILI). The overall results confirm what we already know: RIDTs are helpful in providing a rapid result for many patients, but false-negative results are common. Pooled sensitivities for detecting Influenza A using RIDTs were 54%, compared with 80% for DIAs and 91.6% for NAATs. Pooled sensitivities for detecting Influenza B were similar: Using RIDTs, they were 53%, compared with 77% for DIAs and 95% for NAATs. Pooled sensitivities generally were higher in pediatric patients compared with adults. Keep in mind that false negatives are common when flu is more frequent in your community. False positives are more common when flu is less frequent.

The California Department of Public Health and the CDC have recommended that, regardless of the results of prior rapid influenza testing, empiric therapy with a neuraminidase inhibitor should be administered promptly to patients hospitalized with ILI or suspected influenza, and not necessarily discontinued simply because an RIDT result may be negative. NAAT testing or RT-PCR testing should be performed in all suspect cases, and many hospitals maintain an RT-PCR panel for respiratory virus, which can be helpful. Although treatment has shown the greatest benefit when initiated within 48 hours of onset of illness, evidence supports the administration of antiviral therapy when begun later than 48 hours in those hospitalized with severe flu.