Garth Rattray | A little on COVID-19 antigen testing

Testing is an invaluable tool in the fight against the SARS-CoV-2 virus, which caused the COVID-19 pandemic. Ideally, if widespread testing were done, we could pursue the virus and not just pursue the disease. It is said that more testing would make the picture clearer. We could get ahead of the dreaded infection waves and flattened the curve. Constraints to widespread testing include economic shortfalls, logistical bugbears, and the need for trained personnel and testing apparatus.

COVID-19 testing is needed for diagnosing individuals, surveillance of communities and populations, isolation, quarantining, work and travel, and deciding on the need for lockdowns. The testing currently available in Jamaica are the real-time reverse transcription polymerase chain reaction (RT-PCR) test, the antigen test and the antibody test. Of the tests currently available, the PCR remains the gold standard. PCR testing for COVID-19 has been available in Jamaica since February 19, 2020. Of the PCR tests, University of Sussex senior lecturer in microbiology Dr Edward Wright said, “They detect the genetic information of the virus, the RNA. That’s only possible if the virus is there…”

PCR tests are very sensitive and very specific for detecting the RNA of the SARS-CoV-2 virus. Because they detect the antigen (the virus), and not our reaction to the infection (our antibodies), they are capable of making a definitive and early diagnosis. However, the PCR may return positive results weeks after our bodies have cleared the infection because they may be detecting viral fragments and not viable/infectious viruses. Because of this, the PCR test is no longer depended upon to assess whether patients are no longer infectious. In mild cases, this determination is made by adding 10 days to the time from the appearance of symptoms, plus three or four clear days with no symptoms that could be attributable to COVID-19. In moderate and severe cases, 20 days are initially added. Although it is extremely accurate and useful, the PCR test is complex (has multiple stages), requires: properly trained individuals, expensive equipment and reagents, suitable locations for the equipment, suitable places for testing, appropriate PPEs, and the turnaround time is hours to days.

Easier, Cheaper Test

On the other end of the spectrum is the antibody (serology) test. This test is far easier and cheaper to perform, but is of no help in diagnosing current infection with the SARS-CoV-2 virus. It detects the antibodies that we eventually produce to fight the virus. It is believed that by the time antibodies become detectable (in a week or two), the virus would have already been cleared. This test cannot tell us who is infected; it will therefore only tell us who was infected. But because immunity to SARS-CoV-2 appears to be either very brief or relatively short term, the antigen test can’t predict who will remain immune or resistant to recurrent infections over the long haul. Ostensibly, it will be utilised to ascertain how long the immunity by vaccination lasts.

The rapid antigen test is best used in individuals when their viral load is highest. In other words, it is best done in symptomatic (sick) individuals. Therefore, a negative rapid antigen test should not be assumed to be accurate. In that situation, only a PCR will bring clarity. The sensitivity of the rapid antigen test is listed as ‘moderate-high’, but its specificity is listed as ‘high’. In other words, if it is positive, the accuracy approximates that of the PCR test and, therefore, can be depended on. Because of these characteristics, the rapid antigen test is widely used and has been in Jamaica since November 23, 2020.

In the inaugural Ministry of Health and Wellness monthly briefing for healthcare personnel, the rationale for deciding on rapid antigen testing was outlined as “increase accessibility, decrease turnaround Time, decrease Costs, expand testing services, build laboratory capacity, and facilitate public-private partnerships”. This test may be done in private offices, as long as the rigorous safety protocols are adhered to and appropriate PPEs are worn.

The need to reopen economies has made increased testing necessary. The rapid antigen test is the most popular, but it must be remembered that false negatives are possible, therefore effective control measures must never be abandoned.

Garth A. Rattray is a medical doctor with a family practice. Email feedback to columns@gleanerjm.com and garthrattray@gmail.com.