2.3. COVID-19 Neutralization Antibody Titer Assessment
CONSOLIDATION
The titer of COVID-19-neutralizing antibodies was assessed at baseline and after 1 and 6 months using the iFlash-2019-nCoV neutralization antibody (Nab) assay (iFlash 1800 Chemiluminescence Immunoassay Analyzer Consolidation for Automation, Shenzhen Yhlo Biotech Co., Shenzhen, China). The iFlash-2019-nCoV Nab test is a competitive immunoassay utilizing CLIA as follows:Briefly, the serum samples were placed on a sample rack in the sample loading area, and a reagent pack with 2019-nCoV Receptor Binding Domian (RBD) antigen (30KD)-coated paramagnetic microparticles and acridinium ester-labeled Angiotensin Converting Enzyme 2 (ACE2) conjugate was placed in the reagent loading area. Under a magnetic field, magnetic particles were adsorbed to the wall of the reaction tube, and unbound materials were washed away using the wash buffer. The pre-trigger and trigger solutions were added to the reaction mixture. The resulting chemiluminescent reaction was measured in relative light units (RLUs). Using the ROC curve approach, the cut-off value of 10.00 arbitrary units (AU)/mL was determined for the iFlash-2019-nCoV Nab reagent set. The results were obtained using a calibration curve that was constructed using a four-point calibration and a master calibration curve provided via the QR code on the reagent. The range of measurement was 4–800 AU/mL. At 4 AU/mL, values below the lower limit of the measurement range were recorded. More than 800 AU/mL was used to report values above the measurement range.