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Weldon explains fine line in false negatives

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The biochemist in charge of the main coronavirus screening laboratory said last night the tests used on the island were “very sensitive”.

Carika Weldon added the key to a reduction in false results was to get the right sample from patients.

Dr Weldon, who runs the Government's new Southside testing lab, was discussing the accuracy of the polymerase chain reaction — PCR — tests being used on the island, which have produced 3,451 negative and 118 positive results so far.

Dr Weldon was asked by The Royal Gazette how many of the results — of which less than 4 per cent have come back positive — were likely to be false.

She said: “It's not just to do with the sensitivity of the test. Actually PCR, if you get a good sample, it will find it. That's kind of the main point.

“Really, when it comes to doing the PCR tests the issue is about when you take the sample from the person.”

Dr Weldon said that included whether it was too early or late in the stage of the infection and if the virus was present in the nasal passages and throat, where the samples are taken.

She added: “So it's not necessarily about the test itself. PCR is very sensitive. It's used in a plethora of different molecular diagnostic applications.”

Genetic tests for Covid-19 — those that look for active infections, such as the PCR tests carried out on the island — can vary in their accuracy.

The lower the sensitivity of a test, the higher the likely number of false negative 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.

David Burt, the Premier, said in March that false negative results could “lull people into a false sense of security and unconsciously lead to more spreading of Covid-19”.

The Royal Gazette asked the Ministry of Health this week:

• For the sensitivity and specificity of the local PCR tests, as percentages

• How many of the negative results were estimated to be false negatives

• How that influenced policymaking

A spokeswoman said: “The test is the gold standard which is real-time PCR.

“The sensitivity and specificity are influenced by the reagents and transport.”

The spokeswoman said the advice to people who got a negative result depended on the individual's circumstances, including whether they had symptoms or not.

She added: “From an epidemiologic perspective, symptomatic persons regardless of test results should self-isolate for at least three days post-resolution of symptoms.

“Asymptomatic persons should follow all physical-distancing guidance as provided to the general public.”

No further responses were received.

Dr Weldon said last night the important thing was to get a sample which contained the virus. She added that she had spoken to an immunologist in the United States this week who was working with the country's Food and Drug Administration to evaluate rapid tests.

Dr Weldon said: “He was saying his stance was to do the PCR test and the rapid test at the same time to get a better picture.

“Sometimes when we see that someone has been positive and then they test negative and then they test positive, people are thinking it's a reinfection.

“But actually what it is, is the PCR test missed it the second time and so it's not necessarily a reinfection. It's not that the test is not as sensitive. It's more about getting the right sample at the right time.”

Michael Ashton, the Bermuda Hospitals Board's chief of medicine, said in a social-media video last month he would be “suspicious” of any negative result where a person had symptoms.

Dr Ashton, an infectious diseases specialist, said: “I think it's important to realise that if you have a cough or fever at this time of year, it's pretty much Covid-19.

“I think earlier in the year we could say that maybe it was influenza or rhinovirus ... But for the most part, I think if you have the symptoms, you have Covid-19 until proven otherwise.

“That's probably enough to make the diagnosis clinically. The testing can confirm the diagnosis, but your actions should be basically the same, irrespective of the test result.

“And if you have those symptoms, even if the test result came back negative, I'd be suspicious of that result.”

Dr Ashton added that people with symptoms could use the online symptom checker at www.bermudahospitals.bm.

He said: “It does include travel in the history, which I think is less relevant now that we've got community transmission at this point.”

Sensitive tests: biochemist Carika Weldon at the government testing facility on Southside (Photograph by Akil Simmons)
Testing at Southside (Photograph by Akil Simmons)

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Published May 07, 2020 at 9:00 am (Updated May 07, 2020 at 6:20 pm)

Weldon explains fine line in false negatives

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