Thought of the Day - September 21 - Testing

I thought today I would reflect a little on the state of testing. It was July when we last discussed the subject (Thought of the Day - July 8 - Developments in Testing). Since that time other than millions of additional tests being performed across the country not must have changed. However, a lot has changed in the understanding of how effective the tests are.

As discussed before, the PCR antigen test (swab up the nose) negative error rate may be as high as 30% thus repeats have been necessary. And while false positives look too low, around 0.1%, in some counties with aggressive testing and tracing this false positive has created a "Case-demic". We have around 500 false positives a week across Canada, these lead to needless testing of thousands more people, which lead to more false positives and so on. Indeed some time ago the US CDC recommended to stop giving tests to asymptomatic patients (they have since reversed this due to public outcry). However, last week Alberta announced the same. They gave two reasons, both false positives (Alberta, who has lead the way Canada in testing rates per capita), recons while they have only had a few hundred false positives this does generate extra testing demand, secondly, we will be moving into the flu season shortly and so many more people will be lining up for testing believing they have COVID-19. These two factors put even more pressure on the resources of the system.

Next, there are additional antigen sample collection techniques:

• The "throat swab" also known as the "gag" test is used in some provinces.

• The "saliva" test also known as the "gargle" test. This is just getting started in BC but only for students.

The question with both of these, while they may be easier to administer, are they at least as accurate as the "nose swab"? While they might make testing easier, and thus help provinces meet testing targets, if their accuracy is suspect they have little value.

Next is antibody serology testing. The big news here is that most people start losing their antibodies several weeks after the infection, and thus timing is everything. This also means that antibody testing cannot be effectively used to know who is "immune", as the lasting immunity is provided by different cells that are not tested for. Thus we have no easy way of understanding how close we are to herd immunity.

So like everything else with this pandemic, nothing is easy.

Cheers

Cliff