Though testing done at the National Microbiology Laboratory in Winnipeg showed evidence of antibodies to H7 flu in his blood, tests conducted at the U.S. Centers for Disease Control did not corroborate that finding.
The Atlanta-based CDC did not spot H7 antibodies in the man's blood, officials of the Public Health Agency of Canada revealed on Tuesday. And an additional test which the CDC runs, called a microneutralization test, also did not find evidence of H7 antibodies.
Dr. Gregory Taylor, Canada's acting chief public health officer, says the results are now deemed inconclusive because two labs found two different results. That can happen, Taylor said, especially with newly developed tests for new viruses.
But Dr. Judith Bosse, the deputy minister responsible for Canada's National Microbiology Laboratory, went a bit further, suggesting the Winnipeg lab's test may have generated a false positive result.
"Probably the initial reaction was what we call a false positive, which is the sad part about serology tests," Bosse, assistant deputy minister for the infectious diseases prevention and control branch, said in an interview.
"And that's why we have to run multiple (ones), is false positives do exist."
Microneutralization tests expose cells infected with a virus — in this case a flu virus with an H7 protein on its outer shell — to blood. If the blood has antibodies to the virus, they should shut down the infection. If there are no H7-specific antibodies, infection would continue unabated.
Bosse said this type of test is "fairly specific."
"That's why we're actually ruling out that this is not a conclusive sample," she said.
Late last week the Public Health Agency revealed that an unidentified American whose plane diverted to Edmonton because he was unconscious had tested positive for antibodies to an H7 flu virus. The man had visited China in late May, and then travelled to India. While there he became severely ill and spent time in an intensive care unit in a hospital.
After he was released he travelled to the United Arab Emirates, where he boarded a plane bound for San Francisco. While en route, he lost consciousness, forcing an emergency landing in Edmonton.
While his in-flight illness may have been diabetes related, he went on to develop pneumonia a few days after being admitted to the Edmonton hospital. An infectious diseases doctor who consulted on his case learned of the man's travel history and ordered tests for a variety of pathogens, including H7N9 flu. China battled a large outbreak of the new bird flu this spring.
There was no evidence the man was currently infected — and therefore he was not infectious — but a blood sample sent to the Winnipeg lab tested strongly positive for H7 antibodies.
Bosse said blood tests designed to detect antibodies to new viruses can be difficult to operate in the early days, when they haven't been validated.
In order to ensure a new blood test works well, a laboratory needs to test it against multiple blood samples taken from people who haven't been exposed to the virus as well as samples from people who were known to have been infected and have recovered. Those latter blood samples are called convalescent sera.
Only by running that kind of validation process can a lab be certain their blood test is catching the positive cases and not cross-reacting with antibodies to other viruses. If a test cross-reacts, it could generate a false positive result.
Bosse said the Winnipeg lab is still trying to get access to convalescent sera from China to validate its blood test. Other countries are in the same boat, she said.
Taylor said the lab is re-examining its H7N9 blood test as a result of the event.
"NML functions as the reference laboratory for all of Canada. So when you have a reference laboratory which has a test which is inconsistent, you can bet that they're looking through that very carefully.''
But Taylor said he did not believe this was a case of a laboratory error.
"Our lab has got the very highest quality standards. They are ISO certified. They do absolutely everything possible," he said. (ISO is the International Organization for Standardization.)
"Could it have been a lab error and totally false? I guess anything is possible, but that's not how I'd characterize it. I'd characterize it as a new test that we can't validate with thousands of serum specimens that came forward as positive."
When Canada believed the man had been previously infected with an H7 virus, it notified the World Health Organization and the countries the man visited during his trip. In the latter case that was to alert them to the possibility he may have been infected with H7N9 and might therefore have passed the virus to people he came in contact with during his travels.
Taylor said the Public Health Agency has followed up to inform the countries that the test result is now considered inconclusive.Suggest a correction