Nation Valley News
EASTERN ONTARIO — False positives for COVID-19 are not included in Ontario’s official case count, according to the office of the province’s top doctor, and those who are told they are positive for the virus should know that determination only comes after further investigation and fallback testing.
That message came Tuesday in the wake of a fevered social media response to earlier comments made by Associate Chief Medical Officer of Health Dr. Barbara Yaffe during one of Premier Ford’s regular live-streamed updates last week. Attempting to explain why asymptomatic teachers shouldn’t be regularly tested as they return to school this September, Dr. Yaffe observed July 30 that “you’ll get false positives almost half the time” when testing a population with low levels of COVID-19. “Doing testing on all the teachers would be a huge amount of resources taken away from the need for quick access to testing when somebody may be symptomatic,” she added.
What the doctor didn’t explicitly mention at the time is that the outcome of one test is not used to determine if an asymptomatic individual is actually positive, in areas where the virus is not prevalent. But she clarified that reality when responding to CityNews reporter Cynthia Mulligan, Aug. 4.
Dr. Yaffe explained that two positive tests are required before an individual is recorded as having COVID-19, in such areas. A positive test followed by a negative would keep that person in the negative column.
A senior communications consultant with the Office of the Chief Medical Officer also reiterated the process for NVN: “When we get a positive result, the PHU [Public Health Unit] investigates it in-depth to confirm it’s a case and only then does the case get reported in iPHIS,” said Catherine Fraser by email, referring to the case’s appearance on the province’s online disease reporting system.
Dr. Yaffe had been standing in for the absent Chief Medical Officer of Health Dr. David Williams last week and early this week. Confusion over her July 30 statement triggered all kinds of online citizen speculation questioning the accuracy of test results and even unfounded allegations that Ontario’s case numbers could be overstated by 50 percent.
See the transcript of Mulligan’s question and the doctor’s reply on Tuesday of this week:
Mulligan: Hi, Dr. Yaffe. I was hoping you could clarify a comment that you made on Thursday. It’s causing quite a furor on social media. I asked you about why teachers wouldn’t be tested in schools as a precautionary measure, and you responded by saying in part that just under 50 per cent of tests in low risk areas were false positives. Could you explain that further for me, because I really don’t understand. Are you saying that in areas that are out of hype zones that there are a lot of false positives?
Dr. Yaffe: So, yes, I understand that there’s some lack of clarity. We are talking about a test that is a screening test, basically. And so we’re testing… If we are testing people who we have a population that has no COVID or very little prevalence is extremely low, and you test somebody, and you get a positive… Actually, in fact I think over 50 per cent of those positives will be false positives.
So in those instances, there’ll be a second test recommended. The second test will often then be negative. So that will be ruled out as a case. If the second test is positive, then it will be considered a case. If the test is done in a population where there is COVID as the prevalence or how commonly infection is goes up, the positive predictive value of the test goes up. So that is, what are the chances if you have a positive test, what are the chances that that person actually has the infection? The chances go up as the prevalence of the infection goes up. So it depends on how common the infection is, and it also depends on the.. How good the test is, the sensitivity, specificity. So we have a pretty good test. But if we’re doing it in a population where COVID is rare the positive predictive value is very low. That’s why it has to be repeated, and if it’s false, it will not be reported in iPHIS. It will not be counted as a case.
Mulligan: OK. Thank you. So I’m assuming it’s not falsely inflating the numbers.
Since the start of the pandemic, Ontario has recorded 39,897 cases of COVID-19 — with 36,024 of those now resolved and 2,783 deaths. Sixty-six people are currently in Ontario hospitals because of the virus.