Flu screening tests that are now available at area pharmacies may be practical, but can also miss a flu diagnosis, cautions an infectious disease expert.
Shoppers Drug Mart pharmacies launched a province-wide flu screening pilot project on Jan. 5.
Pharmacists screen patients for flu symptoms and administer a nose swab if they meet the screening criteria. The test screens for influenza A and B. Results are available within 10 minutes.
Four Shoppers Drug Mart pharmacies in St. Albert offer the test, which costs $25.
The tests offer early detection of influenza and subsequent treatment, which is especially important for high-risk populations, says Shoppers Drug Mart in a release.
For immunocompromised patients, an early diagnosis provided by a rapid flu test would be useful, says Dr. Lynora Saxinger, associate professor in the division of infectious diseases of the department of medicine at the University of Alberta.
They are also important for outbreak management and where lab resources are limited. But the amount of benefit it will have for the general population is not as clear-cut, she says.
"If we know the influenza season has started and it's rolling, the vast majority of people who come in with fever and cough will have influenza. A test might not make as much difference in deciding what to do in that setting," says Saxinger.
The accuracy of rapid screening tests is also an issue.
"They're by and large pretty reliable when they are positive, the questions start coming when they are negative (and) if you can believe that they are really negative," she says.
Rapid influenza diagnostic tests are not as sensitive and reliable as viral cultures and reverse transcriptase polymerase chain reaction tests done in the lab, which are the gold standards for detecting influenza, explains Saxinger.
The number of false negatives and false positives change depending on how common the disease is in the community.
"If it's during influenza season, the likelihood that a positive is a true positive is much higher. Out of influenza season, the likelihood that a positive is a true positive drops," says Saxinger.
"Missing a diagnosis would be a bigger risk than misdiagnosing."
False negatives
The rapid influenza diagnostic test being used by Shoppers Drug Mart pharmacies goes by the name of Veritor, manufactured by Becton, Dickinson and Company.
"The studies the company used to register the test quote about an 80 per cent sensitivity, which is quite good. But then we know that tests don't always behave the same in real life settings," says Saxinger, adding that the test has showed variable sensitivity in other studies. In one meta-analysis, sensitivity was as low as 60 per cent.
Health Canada has approved the Veritor screening device for all ages and there are fewer false negatives compared to other on-the-spot tests, states an email from Shoppers Drug Mart to the Gazette.
"Any patient who thinks they might have the flu can discuss the risks, benefits and appropriateness of the screening with one of our pharmacists and make an informed decision about the screening on that basis," it reads.
After taking the test, patients will receive a record of their results.
Pharmacists in Alberta have the ability to prescribe treatment if appropriate and make recommendations for symptomatic relief of the flu, but they cannot prescribe anti-viral medications.
Saxinger says she can see the practical appeal of the screening test.
"If you're felling ill you may not want to face a line-up in the emergency room; you just want to know what's going on."
Saxinger says she doesn't advise against the test, but hopes the screening is done properly – with safety and infection control measures in mind – as well as thorough patient counselling about the risks and the results.
"I would want to caution people that the test might miss influenza. But if it's positive, you're pretty sure it's positive."
Shoppers Drug Mart is sharing information about the pilot with the province, but officials will have to see the results of the project evaluation before determining if there is benefit, stated an email to the Gazette provided by Alberta Health.
"Alberta Health will continue using our own surveillance system to monitor what is happening this season. Our advice to Albertans continues to be: get immunized, wash your hands, practice cough etiquette, and stay at home if you are sick."
The tests will be offered the duration of this flu season, says Shoppers.
What does the research say?
A meta-analysis of 159 studies evaluating the accuracy of rapid influenza diagnostic tests published by Canadian researchers in 2012, found that screening tests are better at detecting influenza A than influenza B.
The tests also perform better in children than adults (13 per cent higher sensitivity) and can be used to confirm the flu, but not to rule it out.
The routine use of rapid flu tests could mean significant improvements in patient care from triage in emergency rooms to community clinics, say researchers.
