Health ministry dismisses notion of false results
The coronavirus tests used in Bermuda are highly unlikely to produce false results, the health ministry said.
A spokeswoman explained: “If a negative sample gets contaminated with a positive sample, you can potentially have a false positive, but there are many health and safety checks in place in labs to prevent this — for example, cleaning of the instruments in between dealing with each sample and the use of UV light to decontaminate any work areas and machines.
“We are confident and certain that our labs follow all such measures and that no contamination has occurred.
“In addition, there are a number of internal test controls performed with each batch to ensure there is no cross-contamination occurring.
“There is one during the extraction process and another during the PCR set-up process. Each have to be negative for any Sars-CoV-2 fluorescence to be valid for analysis.”
The spokeswoman outlined details on the polymerase chain reaction swab tests after a question from The Royal Gazette on whether any of the island's elderly care-home residents who recovered after they got a positive result may not have had the coronavirus in the first place.
Genetic tests for the coronavirus — those that look for active infections, such as the PCR tests — can vary in their accuracy.
The lower the sensitivity of a test, the higher the probable number of false clear results.
The specificity of a test indicates the likely number of false positives.
Both sensitivity and specificity are expressed as a percentage, so, with a test with 95 per cent sensitivity, 5 per cent of negative results would be expected to be false.
The health ministry said the real-time PCR test used on the island was the “gold standard”, but has not given the test sensitivity and specificity.
The spokeswoman said: “Regardless of what population is being tested, the sensitivity and specificity is determined by the test itself and is affected by different variables such as the time of collection.”
She added variables included the incubation phase of the patient and the sample type — nose and throat, mouth and throat, or a sputum test.
The information was shared after David Burt hit back last week at social-media comments that highlighted the high percentage of clear test results.
The Premier said: “It's nonsensical to say that because we have so many negative test results that somehow something is not good about that.”
A total of 12,363 tests had been carried out by yesterday, with 12,217 clear and 146 positive.
Carika Weldon, the biochemist in charge of the Government's screening laboratory at Southside, said in May the tests used here were “very sensitive” and a “good sample” would find the coronavirus.
The health ministry spokeswoman said: “If there is no virus present, this means that there is no RNA to convert to DNA and no DNA to amplify.
“Therefore, there is no exponential increase of fluorescence to detect on the machine.
“This makes getting a false negative with the real-time PCR test impossible, assuming that nothing has corrupted the sample.”