A rapid, point of care red blood cell agglutination assay for detecting antibodies against SARS-CoV-2

The COVID-19 pandemic has brought the world to a halt, with cases observed around the globe causing significant mortality. There is an urgent need for serological tests to detect antibodies against SARS-CoV-2, which could be used to assess the prevalence of infection, as well as ascertain individuals who may be protected from future infection. Current serological tests developed for SARS-CoV-2 rely on traditional technologies such as enzyme-linked immunosorbent assays (ELISA) and lateral flow assays, which may lack scalability to meet the demand of hundreds of millions of antibody tests in the coming year. Herein, we present an alternative method of antibody testing that just depends on one protein reagent being added to patient serum/plasma or whole blood and a short five-minute assay time. A novel fusion protein was designed that binds red blood cells (RBC) via a single-chain variable fragment (scFv) against the H antigen and displays the receptor-binding domain (RBD) of SARS-CoV-2 spike protein on the surface of RBCs. Upon mixing of the fusion protein, RBD-scFv with recovered COVID-19 patient serum and RBCs, we observed agglutination of RBCs, indicating the patient developed antibodies against SARS-CoV-2 RBD. Given that the test uses methods routinely used in hospital clinical labs across the world, we anticipate the test can be rapidly deployed with only the protein reagent required at projected manufacturing cost at U.S. cents per test. We anticipate our agglutination assay may find extensive use in low-resource settings for detecting SARS-CoV-2 antibodies.

Functional activity of intrinsic factor measured by using solubilized receptor protein.

The traditional radioimmunoassay for gastric intrinsic factor, in which this protein is measured on the basis of immunoreactivity rather than function, is of no value for identifying intrinsic factor that binds cobalamin but does not bind to the ileal receptor site, or for detecting animal intrinsic factor, which does not cross react with human intrinsic factor. Accordingly, we have applied a radioassay for the intrinsic factor receptor protein to measure the functional activity of intrinsic factor in gastric juice. The receptor protein reagent was partly purified from guniea pig ilea and its interaction with intrinsic factor--CN[57Co]-cobalamin was determined by precipitation with sodium sulfate at a final concentration of 150 g/L. Results of this assay were comparable with results obtained for intrinsic factor by radioimmunoassay. The receptor protein did not bind immunoreactive intrinsic factor that was functionally abnormal. This functional radioassay for intrinsic factor is not species specific and will be of value when specific antiserum to intrinsic factor is not available and when cobalamin malabsorption is to be evaluated in patients who are secreting normal amounts of immunoreactive intrinsic factor.