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False Positive Test Cases

September 4, 2020

False Positive Test Cases

Last month, Sweden’s Public Health Agency announced that two laboratories discovered an RT- PCR kit for SARS-CoV-2 detection had returned around 3,700 false-positive results. The agency expressed concerns that the kits from BGI Genomics could not distinguish between very low levels of the virus and a negative result.

Lars Engstrand, director and professor at Karolinska Institutet/Science for Life Laboratory in Sweden, performed an analysis of the BGI RT- PCR kit for SARS-CoV-2. When asked about the possibility of false-positive results, he explained in an interview that the rate of positive detection can be linked to test sensitivity.

“This test is very sensitive, and this means that you want to catch all the positive cases in the population…” Professor Engstrand said. “When you have many cases, this kit is very good. It will not miss any positive cases. This means that you can rely on the results, but you also have a chance to get ‘false positive.’ But when the situation changes, and you have a very low number of infected individuals in society, as we have now, then you can lower the sensitivity and be sure that you pick up the right positive cases so you have not so many false positive cases. This is the situation now and we have now changed the threshold or the sensitivity of the test, so now it works perfectly fine in Sweden. We can rely on the test today.”

According to Reuters, “The Swedish labs that evaluate the tests have adjusted their methodology. The agency could not say how many tests had been done using the kit but only a minority of cases appeared to be involved, when the virus is at very low levels.”

Decisions made about the threshold may affect the interpretation of the results, which changes in accordance with the situation. Professor Engstrand explains: “With all tests, it’s a balance if you want to have high sensitivity or not. And in the beginning of the pandemic in Sweden, that was good. Now we don’t need that high sensitivity. We can actually be more precise and don’t risk to catch the false-positive cases. And today we have the situation with this kit.” Despite the special situation of COVID-19, “no test is 100 percent specific or sensitive,” he added. “That means every test has this kind of balance. You have to find out what this balance should be. And this is what we’ve done now in Sweden, for this test.”

Professor Engstrand also assured the public that “this test has been used in many, many countries. And in our hands today, it works perfectly fine. We can validate with other kits and other labs and we have 100 percent agreement between the answers.”

In a response to Sweden’s concern, BGI Genomics stated that the “high positive rate” was due to the amount of virus inside asymptomatic patients, and different countries and regions define asymptomatic patients differently. Different strategies have been implemented around the world. As a country with high population density, China prefers to implement stringent rules, conduct thorough tests, and set high sensitivity to avoid any blind spots and possible infections.