The xMAP SARS-CoV-2 Multi-Antigen IgG Assay (Luminex, Texas; United States) Research Use Only (RUO) kit version 1.00 was used. This assay tests for IgG antibodies in human serum against the SARS-CoV-2 antigens: N, RBD and S1 (Supplementary Material A table 3). The assay is a Luminex multiplex assay using Luminex MagPlex beads. The assay was completed following the manufacturer's instructions for use package insert (Supplementary Material A.3). The 50 samples were tested in a 96 well round-bottomed plate and positive and negative control samples provided in the test kit were included. The plate was then read using the Luminex 200 analyser with a provided xPONENT software protocol.
Individual sample results were considered acceptable when: IgG control bead determined as ‘present’ (MFI > 2500 call threshold), IgA and IgM control beads determined as ‘absent’ (MFI < call threshold determined by software) and background bead determined as ‘passed’ (MFI < 700 call threshold). Samples were considered SARS-CoV-2 IgG antibody positive when their MFI values for both Nucleocapsid bead and RBD bead were > 700 call threshold for each bead. Each run was considered acceptable when both negative and positive control samples met individual sample requirements as above and provided negative and positive SARS-CoV-2 IgG results respectively.
Individual sample results were considered acceptable when: IgG control bead determined as ‘present’ (MFI > 2500 call threshold), IgA and IgM control beads determined as ‘absent’ (MFI < call threshold determined by software) and background bead determined as ‘passed’ (MFI < 700 call threshold). Samples were considered SARS-CoV-2 IgG antibody positive when their MFI values for both Nucleocapsid bead and RBD bead were > 700 call threshold for each bead. Each run was considered acceptable when both negative and positive control samples met individual sample requirements as above and provided negative and positive SARS-CoV-2 IgG results respectively.