Most labs utilize rapid, chromatographic, immunoassays during the winter months when influenza becomes a predominant health concern. Rapid tests have value as part of a physician's process to confirm influenza, but they need to be used correctly and at the right time: 24 to 72 hours after the onset of patient symptoms (CDC guideline).
If you manage a hospital lab or a physician's office lab, there are a number of tips that help improve performance with rapid test assays.
The BD™ EZ Flu A+B assay is one of many rapid test kits that are commercially available in the United States to test for the presence of the influenza virus. Here are a number of good practices that are pertinent to influenza testing, in general, but may be applicable to any number of different test kits on the market.
The clinical diagnosis of influenza should be based on solid clinical signs and symptoms, including but not limited to:
- Sudden onset of symptoms
- A high fever of over 101^0F
- Muscle aches and pains
- Extreme exhaustion
Rapid test kits generally confirm the presence of disease in an individual, but they should not be used as a screening test for the presence of the disease in a population. All negative tests should be confirmed by cell culture because negative results do not preclude influenza virus infection and should not be used as the sole basis for treatment or other management decisions.
Rapid tests typically have a specificity in the range of 95% and should be used during the “influenza season,” as the occurrence of a false-positive result will be significantly reduced when the prevalence of influenza is higher. It makes little sense to use the test assays during the summer or at times when it is known that seasonal influenza has run its course and prevalence is low.
Appropriate specimen collection is so important, yet inappropriate specimens are often collected, tested, and reported. Poor specimens = poor results. In the U.S., preference surveys show that the public prefers anterior nasal swabs (ANS) as the specimen of choice. The fact is that influenza replicates in the nasopharynx. Nasopharyngeal swabs, washes, and aspirates yield far higher viral loads and are better specimens than ANS. But, the adult public dislikes the apparent discomfort of nasopharyngeal specimens (whether performed on themselves or others) and are often displeased at the torment seemingly inflicted upon squirming children and infant populations. In Europe, Australia, and New Zealand, nasopharyngeal swabs, washes, and aspirates are the only acceptable specimen types tested.
Adults tend to wait too long to seek testing and treatment. Don't test adults if they are four or more days into their illness. Children continue to produce copious amounts of drainage, and they can be tested for a few days more than the cut-off for adults.
False-negative results can also occur during the “influenza season,” and can usually be attributed to an inadequate or poorly collected specimen or low viral load.
For children, respiratory syncytial virus (RSV) season can overlap with seasonal influenza, so a decision to test for both viral entities may be necessary. There are rapid RSV test kits available.
All of the rapid influenza test kits usually will provide a test result in 15 minutes or less. Most of the marketed tests can identify and distinguish between A and B influenza virus, but cannot distinguish viral subtypes.
During test performance, ensure that full drops of reagent are added to the test wells, not half-drops or partial drops with bubbles. Ensure that each step of the procedure is allowed the full time allotment according to the package instructions, and don't short-cut the process. Read the manufacturer's package insert to determine if you can use transport media for the specimens, as there is a possibility of dilution of the viral load and a potential false-negative result. Not all transport media work well with all kits, so it is important to use transport media that has been tested by the manufacturer and found to be acceptable. Alternate media would require testing to ensure that is compatible with the assay.
If used correctly, rapid tests for flu can confirm a clinical diagnosis of influenza. And the sooner a diagnosis is made, the sooner appropriate therapy can be administered.
Elliot L. Rank, PhD, D(ABMM), is Director of Scientific Affairs at BD Diagnostic Systems.
