Processing and analysis of serology
All serological samples were processed in a single laboratory (Core Laboratory in the Clinical Research Centre, University College Dublin), in a blinded fashion using three different assays on three different platforms: the electrochemiluminescence immunoassay (14) on an automated Roche platform Cobas® e411, and the chemiluminescent microparticle immunoassay (CMIA) (SARS-CoV-2 IgG 75 assay; Abbott Laboratories, IL, USA) on Architect i2000SR and Alinity and the chemiluminescent microparticle immunoassay (CMIA) (SARS-CoV-2 IgM assay; Abbott Laboratories, IL, USA) on Architect i2000SR Plasma samples were processed immediately after collection and stored at -80°C prior to analysis.
The Elecsys anti-SARS-CoV-2 serology assay is a sandwich immunoassay intended for the detection of IgM and IgG antibodies to SARS-CoV-2 in human serum and plasma. 140 μL (20uL +120uL dead volume) of sample was used in the assay. Results were determined automatically by the software by comparing the electrochemiluminescence signal obtained from the reaction product of the sample with the signal of the cut-off value previously obtained by calibration with ACOV2 Cal1 containing human serum, non-reactive for anti-SARS-CoV-2 antibodies, and ACOV Cal2 containing human serum reactive for anti-SARS-CoV-2 antibodies.
Plasma samples were also run on the Abbott Architect i2000SR and the new Alinity instruments using the Abbott SARS-CoV-2 IgG assay. The assay is a two-step immunoassay using CMIA technology for qualitative detection of IgG in human serum or plasma, raised against the nucleocapsid protein of SARS-CoV-2. The Architect requires a volume of 75µL of serum or plasma (25uL+50uL dead volume). Qualitative results and index values reported by the instrument were used for analysis (15). A signal/cut-off (S/CO) ratio of ≥1.4 was interpreted as reactive. Calibration was performed and positive quality control S/CO 1.65–8.40 and negative quality control S/CO ≤ 0.78 were fulfilled prior to analyses of patient samples. Within-day imprecision assessment was performed using QC material.
IgM Plasma samples were run on the Abbott Architect i2000SR. A signal/cut-off (S/CO) ratio of ≥1.0 was interpreted as reactive.