Antibody testing for virus ‘set to fail’ due to large volume of false positives


An attempt to establish the number of people here who have contracted Covid-19 to date faces failure due to the low prevalence of the virus here, with 5% of the population set to give a false positive for coronavirus antibodies.

The first study aimed at establishing how prevalent Covid-19 has been across the country as a whole is due to start in Dublin and Sligo later this month, according to the HSE.

It said that the test will involve a random sample of several thousand people, who will be invited to submit a blood sample.

However, a recent seminar given by the Irish College of General Practitioners for its members heard that there is a mistaken belief that “there is a reliable blood antibody test to see if you have had Covid”.

The seminar, given by a consultant immunologist at a Dublin hospital last week, heard that the test to be administered by the Irish population is likely to return with a false positive rate of 5%.

Taking Ireland’s population of 4.9m that would see a return of roughly 240,000 false positive cases involving people who have not actually been infected with the virus.

Assuming those figures are taken at face value, it could lead to a situation in which six out of seven people are falsely marked as having at some stage contracted coronavirus.

The ICGP had not responded to a query at the time of publication as to whether or not it considers the antibody test to be fundamentally unreliable.

The issue of false positives could explain why the National Virus Reference Laboratory, which is charged with conducting the initial seroprevalence study, has opted to conduct the test in the Leinster and Connacht counties, as these counties represent the regions with both the highest and one of the lowest prevalence rates for the virus, thus enabling a comparison to evaluate how effective the antibody test is in practice.

The same seminar also heard that there is no evidence that presenting with antibodies to the virus indicates that the person in question is either immune to the virus or that they are no longer infectious.

Recently the National Public Health Emergency Team (NPHET) heard from its expert advisory group that people who have recovered from the virus can thereafter be considered to be immune for a period of three months.

Seroprevalence [the level of a pathogen in a population, as measured in blood serum] has come to be interpreted as a means of allowing the public to return to a semblance of normality while living alongside the virus.

One GP told the : “It’s just a mess. People are holding their breath for things that are going to make us be able to return safely to normal life. I don’t think we’re there yet.”

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