Performance of an automated chemiluminescence SARS-CoV-2 IG-G assay

Abstract

Introduction: We describe our evaluation of the Abbott SARS-CoV-2 IgG assay on the Architect immunoassay analyser.

Methods: We assessed assay precision, sensitivity, specificity, positive/negative predictive values (PPV/NPV), cross-reactivity (influenza/dengue/hepatitis B and C/rheumatoid factor/anti-nuclear/double-stranded DNA/syphilis) and sample throughput in samples from real-time polymerase chain reaction (RT-PCR) positive patients/healthcare workers (HCWs)/pre-pandemic samples. We compared the cut-off indexes (COIs) between all control samples (HCWs and pre-pandemic) to generate an optimised COI limit for reactivity.

Results: The assay specificity was 99.8% (n = 980) and sensitivity was 45.9-96.7% (n = 279). When tested ≥ 14 days post-positive RT-PCR (POS), the PPV/NPV was 96.4%/99.8%. The difference between the COIs of HCWs/pre-pandemic samples was small (0.01, p < 0.0001). There was minimal cross-reactivity with other antibodies. A lower COI limit for reactivity (≥0.55, using the 99th percentile COI of our controls and ROC analysis) improved diagnostic sensitivity, especially at 0-6 days POS (45.9-55.8%), with a small decrease in specificity (98.9%). The assay throughput was 100 samples in 70 min.

Conclusion: The Abbott SARS-CoV-2 IgG assay shows excellent performance in patients ≥ 14 days POS. The difference between the COIs of HCWs and pre-pandemic samples was numerically small. A lower COI limit improves assay sensitivity with a slight decrease in specificity.

Keywords: Antibodies; Assay evaluation; Reactivity index; SARS-CoV-2.

Fig. 1

Mann-Whitney U comparison between 2 populations of controls. (* COIs using logarithmic scale) (Abbreviations: HS: Health screen samples, HCW: Healthcare worker samples).

Fig. 2

Serial COIs of RT-PCR positive patients over days post-positive RT-PCR. Each curve labelled by a Latin letter represents an individual patient. (*COI values displayed on logarithmic scale.) (Abbreviations: COI: Cut-off index, POS: days post-positive RT-PCR.)