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SARS-CoV-2 Antibody test and specificity

Updated: May 6, 2020

"Testing, testing, testing" has been the mantra from the head of the World Health Organisation, Dr. Tedros Adhanom Ghebreyesus. The knowledge that SARS-CoV-2 infection can lead to no or mild symptoms for many has led to an increased clamour for more testing amongst those around the world undergoing self-isolation and lockdowns.

The question on many minds is "Have I already had it?"  At the same time, public health officials need greater clarity on how far the infection has spread, and greater certainty over the infectivity profile and case fatality rates of the virus.

  The most commonly used tests use swabs to detect different parts of the viral genome, but because these require a lab to amplify the extracted RNA, tests are slow to process and there are critical supply issues over key reagents.  Rapid expansion of lab capacity is helping but focus has shifted increasingly to antibody, or serological testing.

Singapore first tested a prototype in February 2020, and at about the same time, the China's State Key Laboratory of Respiratory Diseases  started producing a test that detected IgM antibodies in 15 minutes at room temperature.  A key advantage is that IgG antibodies persist in the body long after the virus has been eliminated, providing evidence of prior infection.

Chinese research companies such as WondFlu are now actively exporting approved serological testing kits approved by  the Chinese National Medical Products Administration  around the world, as well as an increasing number of companies in UK, Europe, USA and elsewhere developing similar kits either in collaboration or independently.  Supply can in no way compete with demand at this stage, and as we heard yesterday, Public Health England is expecting to purchase "hundreds of thousands" of kits as soon as practical. But the challenge is not only supply.  It relates to the concept of specificity.

In China, such serological tests are being used for triaging purposes on symptomatic individuals, before carrying out more definitive lab-based RNA-PCR tests.  For any diagnostic test, you not only want to have high sensitivity, the probability of a positive test result from a previously infected individual.  You also would like a high specificity, the probability of a negative test result from someone who has not yet been infected.

The UK's Chief Scientific Advisor, Sir Patrick Vallance, has highlighted the difficulties of evaluating the different serological tests on offer.  If the specificity is too low and tests are carried out in isolation, an unacceptably large number of individuals without prior SARS-CoV-2 infection will get a positive result and believe that they are immune from further infections.  Given the exponential spread of this virus and the importance of maintaining social distancing and self-isolation to suppress further outbreaks, such mistakes could lead to confusion, mistrust and ultimately new outbreaks.  The need for high specificity is key.

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