Are false positives from Cellex’s Coronavirus antibody test systemic?
The FDA just approved the first antibody test for the Coronavirus, made by Cellex. The test has a significant false positive rate though (see here). Now for a lot of tests, false positives can be reduced by retesting positive cases, as the probability of two false positives for the same person is usually low. But this tweet by epidemiologist Zachary Binney raises the possibility that retesting may not help in this case:
Running the test 2x & only telling someone they're immune if they get 2 +s might help reduce false +s, depending on the error source. If it's anything systemic - say it's detecting antibodies from a similar virus that don't grant immunity to COVID-19 - no good.
My question is, is anything known about the sources of error for Cellex’s antibody test, and whether it’s anything systemic along the lines that Binney outlined?
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You can see all the tests being produced here www.centerforhealthsecurity.org/resources/COVID-19/Serology-based-tests-for-COVID-19.html
Many of the tests share similar sensitivities and specificities.
The Cellex test looks at both IgG and IgM antibodies.
RDT, lateral flow assay, which detects IgM and IgG to the nucelocapside protein of SARS-CoV-2. The sensitivity is 93.8% and specificity is 95.6%, when tested at 2 Chinese hospitals in a total of 128 COVID19 positive patients, and 250 COVID19 negative patients (as detected by RT-qPCR).
So they are approximately picking up 4 per 100 as positive that don't have PCR evidence of infection. This could mean that the test is actually more accurate because some of these false positives did actually have the disease in the past, or, it is detecting an antibody against another coronavirus like SARS or MERS. They have already controlled for the seasonal coronavirus.
False positives can occur due to cross-reactivity with antibodies from previous infections, such as from other coronaviruses.
This may be less of a problem in the USA population and also if you have a history compatible with infection the more likely the test is a true positive.
It will be more interesting if they can get validity data from a USA population.
Edit: there is a new paper from nature medicine suggesting that the sars-cov-2 might have been circulating for some time, even years before it jumped to become more lethal. This might explain the false positive rates.
www.medscape.com/viewarticle/928150 www.nature.com/articles/s41591-020-0820-9
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