An international consortium led by researchers from IPBS and Oxford University has developed an extremely simple and inexpensive serological test for the detection of antibodies against the SARS-CoV-2 virus, responsible for COVID-19. With a sensitivity of over 90% and a specificity of 99%, this test performs far better than most commercial rapid tests, the majority of which are based on immuno-chromatographic approaches. This work is reported in a manuscript deposited in MedRxiv and submitted for peer review prior to publication.
Detection of antibodies to the SARS-CoV-2 virus is essential to monitor the progress of the pandemic and explore whether the presence of antibodies correlates with protection against COVID-19. The test could also be used to assess the presence of antibodies after vaccination when a vaccine becomes available. Several commercially available tests have been described that are highly effective, but require centralised laboratory facilities that are relatively expensive and therefore not universally available.
The test developed at the initiative of Dr. Etienne Joly, researcher at IPBS, in close collaboration with Prof. Alain Townsend (FRS, University of Oxford, UK) is based on the haemagglutination method. It has a sensitivity of 90% and a specificity of 99% for the detection of antibodies after an infection diagnosed by PCR.
This test is based on the use of a unique reagent: a recombinant protein resulting from the fusion of a single-chain antibody ("nanobody") against a protein, glycophorin, on the surface of red blood cells and the "RBD" domain of the S protein of the SARS-CoV-2 virus. When this protein is mixed with a dilution of a whole blood sample, this reagent will "disguise" the red blood cells as a large virus. If antibodies to the virus are present in the blood, this will cause the red blood cells to hemagglutinate, which can be detected by eye after one to two hours of incubation.
This test does not require any special equipment and can be carried out anywhere. The test reagent alone costs less than one cent per test performed. At this stage, researchers have had one gram of the reagent produced, which is enough to perform more than 10 million tests. They are offering to provide free lyophilised 1 mg samples (10,000 tests) to all laboratories that request them.
This recombinant protein can bind to the surface of red blood cells via the HI4 domain; the red blood cells will then carry the RBD domain of the virus on their surface. If the blood tested contains antibodies to this domain, this will result in the agglutination of the red blood cells, which can be easily detected by eye, as exemplified in the right-hand side of the figure. The test is performed in conical bottomed wells. After one hour of incubation the red blood cells have sedimented and form a red button at the bottom of all wells. The plate is then tilted at 80° for 30 seconds and the non-haemagglutinated red blood cells will form a tear. In the presence of antibodies, the clumped red blood cells remain "knobbed" at the bottom of the well and will not form a tear. For each sample, a negative control performed without the IH4-RBD reagent shows tear formation (right well).
Scientist: Etienne Joly | Etienne.Joly@ipbs.fr | 06 62 24 59 91
Press: Françoise Viala | Communication@ipbs.fr | 06 01 26 52 59