FDA authorises the first Covid-19 antibody test

Cellex wins the race to get the FDA’s buy-in for a blood test to pinpoint immunity.

The FDA has granted emergency use authorisation to a steady stream of diagnostics for active Covid-19. On Thursday it also granted an EUA for a serological or antibody test, capable of identifying people who have had a previous coronavirus infection, and who are therefore likely to be immune to the disease. 

Developed by Cellex, this immunoassay and others like it will be crucial in helping the US recover from the pandemic, since it will allow those with a positive result to go back to work. However, scale-up will be a major challenge for the small private company. More importantly it should be remembered that an emergency authorisation does not amount to a determination by the FDA that a test has been proven accurate.

Cellex’s submission for an EUA is intriguing since in March the FDA said antibody tests did not require this authorisation, and indeed at least one other is already in use in the US. The agency said serological assays may be used without its oversight, provided they are clinically validated and not used as the sole method to diagnose an infection. Cellex clearly thought it was worth getting authorisation anyway – perhaps for marketing purposes.

Cellex’s assay can detect IgM and IgG antibodies against the coronavirus in serum, plasma or whole blood – the latter will most often be used, in the form of a fingerstick. IgM antibodies to the coronavirus are generally detectable in the blood several days after initial infection, the FDA said, while IgG antibodies become detectable later, after the infection has cleared.

The company’s R&D labs are based in North Carolina, where it also has a manufacturing facility – but Cellex also has an FDA-compliant manufacturing site in Suzhou, China. The tests will actually be performed at the point of care. 

Becton Dickinson is also selling a fingerstick antibody test in the US, though without an emergency authorisation. This test, which was developed by BD’s partner BioMedomics, will be available this month. BioMedomics claims sensitivity of 88.7% and specificity of 90.6% for the test. On April 2 BD received an EUA for a different test – one that picks up viral RNA in a sample taken from a patient’s nose or throat. 


One important question is whether these tests are reaching enough patients in the US. According to Leerink analysts, more than 1.7 million molecular diagnostics for the coronavirus have been performed in the country so far, with the number growing by around 100,000-120,000 per day. Consequently the number of confirmed cases has continued to rise: in New York, the positive rate is now 40%.

But the rate of expansion of viral RNA testing is slowing as Clia labs reach capacity. With the outbreak of Covid-19 in the US still far from levelling off, this will be a problem – and granting authorisation to more tests will not alleviate it. The availability of a new molecular test is unlikely to come alongside the opening of a new Clia facility.

Still, the availability of the serological tests is a positive, since they will take up little or no Clia lab space. Moreover, along with being able to return to work, people with proven immunity will be able to donate plasma to be processed into an emergency therapy for active Covid-19 patients. 

But accuracy is the critical point. This has derailed efforts to launch antibody tests in the UK: the country's health secretary Matt Hancock, admitted last week that several tests under investigation had failed to produce accurate results.

Leerink analysts write that tests with sensitivity and specificity of more than 95% would have “significant potential”. It is salutary that the BD test, one of the very few immunoassays for which accuracy figures have been revealed, does not meet this standard.

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