National Influenza Vaccination Week (NIVW) is recognized every year during the first week of December. The CDC created NIVW in 2005 to encourage people to protect themselves against influenza, a serious illness that still claims thousands of lives every year. NIVW is a public reminder that December is not too late to get a flu shot, since flu season is just getting started in the Northern hemisphere. The vaccine can protect you against up to 4 strains of influenza, and it’s especially important for healthcare workers who have contact with vulnerable patient populations.
Laboratorians, of course, have another important role in preventing the spread of influenza: running the tests that detect the virus as it starts to affect a community.
Influenza testing
Rapid antigen detection methods are still the most common type of influenza testing in the average lab. These kits, called Rapid Influenza Diagnostic Tests (RIDTs) use an immunoassay to detect the presence of influenza antigens, and are most sensitive during the first four days after the onset of symptoms. Over ten different RIDTs are approved by the FDA, and kits can be either waived or moderate in complexity.
They’re easy to use and the results are easy to interpret, but it’s important for laboratorians and clinicians to know the limitations of RIDTs when it comes to detecting influenza during an outbreak.
Sensitivity
RIDTs have a high false-negative rate (up to 40% in some cases). This means that many patients presenting with influenza symptoms during flu season will have a negative test in the physician’s office. This is a problem, because most antiviral medications must be administered within the first few days of illness in order for them to have any effect. Depending on an RIDT alone for diagnosis may mean that patients with influenza will miss out on treatment. This is why it’s common for presumptive negative tests to be followed up with a more sensitive method like molecular assay (RT-PCR) looking for influenza virus DNA, especially in patients who are at high risk of secondary infections like pneumonia.
While it’s not possible to eliminate all false negatives, medical and laboratory professionals can often improve the sensitivity of their lab’s RIDT by being diligent about acquiring good specimens from their patients. After all, even the most accurate test is only as good as the specimen it’s given. Different test kits are designed to be used with specific specimens, so it’s critical to read the manufacturer’s directions in detail. Some tests can be run off the eluate from a nasal swab, but some are only approved for deep nasopharyngeal swabs or nasal washes. It’s possible for a laboratory to reduce the number of false negative results by only collecting the appropriate specimen for the kit they are using.
Specificity
Many RIDTs are able to detect the presence of both type A and type B influenza viruses, but only a few can distinguish between them. This can have clinical implications, as influenza A viruses tend to be more virulent and cause more severe symptoms, especially in high-risk populations. None of the currently-available RIDTs can distinguish between different subtypes of the more virulent influenza A.
When there is an outbreak of a specific subtype of influenza A, such as in the H1N1 “swine flu” epidemic in 2009, state and federal health agencies need to know whether a patient’s illness is caused by the new subtype, or another circulating seasonal strain of influenza A. In these situations, only a viral culture will be able to isolate the specific strain for analysis and allow public health agencies to track the progress of the outbreak. Very few labs are equipped for this sort of testing – most labs who find themselves in the midst of an outbreak will be shipping swabs to a state reference lab for follow-up testing.
Despite their limitations, RIDTs are a quick and inexpensive way to detect flu in a patient population. Positive results in high-risk patients, especially the elderly in hospitals or long-term care, allow for rapid deployment of antiviral treatments, which can save lives.