The COVID-19 pandemic has caused significant morbidity and mortality. There is an urgent need for serological tests to detect antibodies against SARS-CoV-2, which could be used to assess past infection, evaluate responses to vaccines in development, and determine 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 have not scaled to meet the demand of hundreds of millions of antibody tests so far. Herein, we present an alternative method of antibody testing that depends on one protein reagent being added to patient serum/plasma or whole blood with direct, visual readout. Two novel fusion proteins, RBD-2E8 and B6–CH1-RBD, were designed to bind red blood cells (RBCs) via a single-chain variable fragment (scFv), thereby displaying the receptor-binding domain (RBD) of SARS-CoV-2 spike protein on the surface of RBCs. Mixing mammalian-derived RBD-2E8 and B6–CH1-RBD with convalescent COVID-19 patient serum and RBCs led to visible hemagglutination, indicating the presence of antibodies against SARS-CoV-2 RBD. B6–CH1-RBD made in bacteria was not as effective in inducing agglutination, indicating better recognition of RBD epitopes in mammalian cells. Given that our hemagglutination test uses methods routinely used in hospital clinical labs across the world for blood typing, we anticipate the test can be rapidly deployed in low-resource settings at minimal cost. We anticipate our hemagglutination assay may find extensive use in low-resource settings for detecting SARS-CoV-2 antibodies.

BACKGROUND Isohemagglutinins (anti‐A and anti‐B) mediate hemolytic transfusion reactions, antibody‐mediated rejection of solid‐organ transplants, and delayed engraftment after stem cell transplant. However, quantification of isohemagglutinins is often labor intensive and operator dependent, limiting availability and interfacility comparisons. We evaluated an automated, solid‐phase and agglutination‐based antibody titer platform versus manual gel testing. STUDY DESIGN AND METHODS Plasma samples were obtained from 54 randomly selected patients. Titers were determined by our laboratoryʼs standard assay (manual dilution followed by manual gel testing) and were compared to results obtained on a fully automated blood bank analyzer (Galileo NEO, Immucor). The analyzer determined immunoglobulin G (IgG) antibodies using solid‐phase and immunoglobulin M (IgM) antibodies by direct hemagglutination. RESULTS Isohemagglutinin titers obtained by manual gel versus the automated assay generally (>80%) agreed within one doubling dilution, and always (100%) agreed within two dilutions. Among O samples, the gel titer and the highest titer obtained with the automated assay (either IgG or IgM) were similar in paired, nonparametric analysis (p = 0.06 for anti‐A; p = 0.13 for anti‐B). Gel titers from group A and group B patients were slightly higher than the highest titer obtained using the automated assay (p = 0.04 for group A; p = 0.009 for group B), although these differences were within the accepted error of measurement. CONCLUSION Manual and automated methodologies yielded similar isohemagglutinin titers. Separate quantification of IgM and IgG isohemagglutinins via automated titration may yield additional insight into hemolysis, graft survival after ABO‐incompatible transplantation, and red blood cell engraftment after ABO‐incompatible stem cell transplant.

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. was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.