The accuracy of a rapid finger-prick antibody test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may be considerably lower than previously suggested, according to a study published in BMJ.

The study suggests that if 10% of people given the test had previously been infected, approximately 1 in 5 positive test results would be incorrect, showing false positive results. These conclusions differ from an earlier study suggesting that the test gives no false positive results.

The findings suggest the test can deliver a sufficient degree of accuracy for surveillance studies of the population, but laboratory confirmation of positive results is likely needed if these tests are to be used to provide evidence of protection from the virus, according to the study.

The AbC-19 Rapid Test uses a drop of blood from a finger-prick to see whether it’s likely that someone has previously been infected with SARS-CoV-2, giving results in 20 minutes without the need to go to a laboratory.

The researchers tested blood samples in a laboratory from 2847 key workers in England during June 2020, such as health care, fire, and police staff. Of these, 268 had a previous positive polymerase-chain reaction (PCR) result, whereas the remaining 2579 had unknown previous infection status. A further 1995 pre-pandemic blood samples were also tested as “known negatives.”

The research team estimated the specificity of the AbC-19 test to be 97.9%, meaning that the 2.1% of people who did not have a previous SARS-CoV-2 infection incorrectly tested positive. Further, they estimated the sensitivity of the AbC-19 test to be 92.5% based on PCR confirmed cases, but considerably low (84.7%) in people with unknown previous infection status prior to antibody testing, according to the study.

The difference is probably due to the test being more sensitive when antibody levels are higher, as people with a positive PCR result tended to have more severe disease and are more likely to have produced more antibodies, according to the study.

In addition, trained laboratory staff noted the test results band was often weak and disagreed on whether the result was positive or negative for almost 4% of AbC-19 tests, implying that test accuracy could be lower still if the test was used at home by members of the public, according to the study.

The study had several limitations, including that the test was evaluated in a laboratory rather than having participants perform the test themselves, which may have overestimated performance, according to the authors. Further, the study included few people over 65 years of age, suggesting the need for further evaluation of the test in older ages when risk of severe coronavirus disease 2019 is higher.

There is also a possibility that other lateral flow devices detecting antibodies to SARS-CoV-2 may also work less well at lower antibody concentrations. This study specifically did not investigate this, but the study authors note that their work “highlights the scope for overestimation of SARS-CoV-2 antibody test sensitivity in other studies in which sensitivity has been estimated only from PCR confirmed cases.”

The study authors recommend further work to clarify the relation between circulating antibody levels and immunity to SARS-CoV-2, noting that a clear message must be communicated to the public that positive results from these assays do not provide evidence of immunity.

REFERENCE
Accuracy of rapid covid test may be lower than previously suggested. BMJ. https://www.bmj.com/company/newsroom/accuracy-of-rapid-covid-test-may-be-lower-than-previously-suggested/. Published November 11, 2020. Accessed November 12, 2020.