How Accurate are Rapid Flu Tests?

Relying on the rapid influenza test may not be your best bet

For some, the holiday season brings the joy of cooler temperatures, peppermint mochas and kindling fires. But for nurse practitioners, the time of year also means the loom and doom of the dreaded influenza season.

Prescribing antiviral medication for the flu has proven to not only reduce a patient’s symptoms and recovery time, but control the spread of the virus within the community as well. However in order for antiviral prescriptions to be most effective, the patient must begin taking them within 48 hours of symptom onset. With such a short window of opportunity for effective treatment, how can providers be confident in diagnosing and treating patients with the flu?

A common method for diagnosis of the flu is based on a combination of assessing a patient’s symptoms and performing a flu test. Testing for the flu can be done via a laboratory diagnostic test or a rapid influenza diagnostic test (RIDT).

While laboratory diagnostic tests are 85 – 100% accurate, results can take up to two days to be produced and often require the patient to visit a diagnostic facility; further delaying treatment of the patient and increasing the risk for the virus spreading to others. In addition, these tests are typically only used to help public health officials determine what strains of influenza are circulating and are not generally used to diagnose and determine treatment options for patients because of the length of time required for results. 

RIDTs, however, are first-line tests that have been approved by the CDC to be used in point-of-care settings. Known for producing quick results in 15 minutes or less with a simple nasal or throat culture from the patient, decisions regarding antiviral therapy, antibiotics and additional testing can be made within minutes of a RIDT, long before results from laboratory diagnostic tests become available. Unfortunately, though convenient, RIDT accuracy varies from 50 – 70%; meaning that in up to half of influenza cases, the swab results give false negatives. It should also be noted that sensitivity of the RIDT is higher in children, between 90% and 95% as they shed more of the influenza virus.

Contributing factors such as the level of influenza activity in a community play a major part in RIDT accuracy. For example, when influenza prevalence is low, such as at the beginning or the end of flu season, the test has a tendency to produce a false positive. But when there is a heavy outbreak of the flu, such as at the height of flu season, the results tend to sway in giving false negatives. Other factors such as using swabs that did not come with the test kit or improper storage or prolonged storage before specimens are tested can also contribute to inaccuracies. 

Although far from perfect, RIDTs can still supplement your judgment in treating patients with flu-like symptoms and a diagnosis of influenza can be confidently made when the RIDT is positive; as although though false negatives are frequent, a positive test is unlikely to be a false-positive.

So if your patient presents with typical flu-like symptoms during the peak of the season, bear in mind that a negative test result does not exclude the diagnosis of influenza. Using your best clinical judgment is likely inferior in accuracy compared to RIDTs, and antiviral treatment should be started without hesitation. A negative RIDT can always be confirmed by a traditional diagnostic test. 

 

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