December 13, 2020 - PCR and Antigen Tests
PCR and Antigen Tests
December 13, 2020
It is Sunday again and that means the data in ADPH’s report was collected on Saturday, a typically light-reporting day for many test centers. That proved true again today, as just 2,790 new cases (incl. 496 probables) were reported. After 5 straight days of daily cases between 3,500 and 4,500, we needed a break. However, it’s not accurate to say there were only 2,790 cases today; the truth is, there are more than 2,790 new cases out there, but only 2,790 KNOWN cases were reported.
For the second straight day, ADPH failed to report the number of daily tests, so I cannot accurately calculate the positivity rates. On Friday, the rate was 32.4%. According to Johns Hopkins, fewer tests have been performed in Alabama than in all but 2 states (Idaho and Iowa) over the last 7 days, so I am confident the positivity rate has not fallen meaningfully since then.
Two days ago, ADPH issued a release, noting that it had implemented new procedures for processing antigen test results that will cause daily probable case totals to increase but will result in fewer “data dumps”. What is that all about, you ask? To answer that question, some nerdy - but very important - background is required. Please bear with me.
PCR and antigen tests serve different purposes but one is not necessarily “better” than the other. PCR tests can detect the presence of the virus before it is contagious, which is useful, but it requires the expertise of a lab, which usually takes longer to get results. Antigen tests have rapid turnaround and are better indicators of contagion. It is, therefore, often the case that a PCR test will come back positive when the antigen test is negative. That can happen in the early stages, for instance, when the virus is present but not yet contagious. Hence, antigen tests are more likely to produce “false negatives”; however, a positive antigen test result is almost certainly a real, infectious case of COVID-19.
Last spring, the CDC decided that only positive cases confirmed by a PCR test would be deemed “confirmed”, whereas positive cases diagnosed with antigen tests would be classified as “probable”. Despite this guidance, most states continued to treat antigen positives the same as PCR positives. Not in Alabama, however, where positive antigen tests were often discounted for political reasons by laymen as something less than a real case - only “probable”.
For reasons I have been unable to fathom, ADPH has had difficulty processing positive antigen results in a timely manner. That’s why ADPH will occasionally catch up on “backlogs” of probable cases. Apparently, ADPH has finally developed a way of tabulating antigen test results so these delays can be reduced. We therefore can expect probable cases to be reported more timely in the future, with fewer “backlogs”.
The important thing to remember is that a positive antigen test result - i.e. a “probable” case - is due the same respect as a confirmed PCR test; both are real cases of COVID-19. There are times when an antigen test is even more useful than a PCR test - for example, if you are experiencing mild symptoms and want to know if it’s COVID or just a cold, then an antigen test may be preferred because of the rapid turnaround and the potential need to isolate.
I hope this explanation has been helpful. Now, the totals:
11/30 - 2295
12/1 - 3376
12/2 - 3928
12/3 - 3531
12/4 - 3840
12/5 - 3390
12/6 - 2288
12/7 - 2335
12/8 - 4436
12/9 - 3522
12/10 - 4735
12/11 - 3853
12/12 - 4066
12/13 - 2790
Two counties continue to stand out - Jefferson with 477 cases and Madison County with 270 cases.