How Accurate Is COVID-19 Antibody Testing?

December 23, 2020

Written by Clay Smith

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Sensitivity and specificity of antibody testing for SARS-CoV-2 in hospitalized patient when measured ≥14 days of illness was 97.6% and 98.8%.

Why does this matter?
What does the presence of SARS-CoV-2 antibody mean? Does this mean a patient is immune and protected? Is the test for antibody accurate at all? This article helps us, at least somewhat, with the last question.

What does it meeeeean?
This was a small, single center case-control study with 60 hospitalized COVID-19 cases, confirmed by molecular testing and clinical symptoms, and 513 SARS-CoV-2 seronegative controls who were either healthy volunteers or other patients with serum sent to the immunology lab for other reasons. Cases had multiple serologies drawn during their hospital stay. The sensitivity of IgG to the SARS-CoV-2 spike protein when measured at day 14 or later was 97.6% (95%CI 92.8% to 99.5%); specificity was 98.8% (97.4% to 99.5%). The higher the IgG titer, the more likely a patient was to develop ARDS. There were a few problems with this study. This was a sample of cases who were all sick enough to be in the hospital. Performance of antibody testing may not be as good in less sick outpatients with COVID-19. In addition, the gold standard test (PCR NP swab) was not done on all the controls, which is not ideal (although some controls’ serum was collected before 2019 – so they certainly didn’t have COVID-19). Despite these concerns, I still find this encouraging.

Source
Clinical Validity of Serum Antibodies to SARS-CoV-2 : A Case-Control Study. Ann Intern Med. 2020 Oct 20;173(8):614-622. doi: 10.7326/M20-2889. Epub 2020 Jul 6.

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