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.