Serologic testing may aid in surveying asymptomatic infection, assessing past SARS-CoV-2 infection prevalence, or serving as an adjunct for COVID-19 diagnosis if used ≥15 days after symptom onset in cases with suggestive clinical presentation but negative or unavailable reverse transcriptase PCR (RT-PCR) results ( 3 – 6). While molecular-based testing is used to diagnose COVID-19 ( 1), serologic testing of antibodies specific to SARS-CoV-2 is used to detect past infection ( 2). First declared a Public Health Emergency of International Concern in January 2020 by the World Health Organization (WHO), COVID-19 has infected over 88 million people globally, causing over 1.9 million deaths as of 10 January 2021 ( ). The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of coronavirus disease 2019 (COVID-19). Lastly, antibody response showed high interindividual variation in level and time of peak antibody titer and trends over time. Imprecision was <2% at 9.06 U/ml across 6 days, the negative quality control (QC) was consistently negative (<0.40 U/ml), the manufacturer’s claimed limit of quantitation of 0.40 U/ml was verified, and linearity across the analytical measuring range was observed, except at the low end (<20 U/ml). The Elecsys anti-SARS-CoV-2 S assay exhibited its highest sensitivity (84.0%) at 15 to 30 days post-PCR positivity and exhibited no cross-reactivity, a specificity and PPV of 100%, and an NPV between 98.3% and 99.8% at ≥14 days post-PCR positivity, depending on the seroprevalence estimate.
Clinical evaluation included assessing clinical sensitivity, specificity, cross-reactivity, positive predictive value (PPV), negative predictive value (NPV), and serial sampling from the same patient. Analytical evaluation included assessing linearity, imprecision, and analytical sensitivity.
Specimens from 167 PCR-positive patients and 103 control specimens were analyzed using the Elecsys anti-SARS-CoV-2 S assay on the cobas e411 (Roche Diagnostics). We evaluated the quantitative Roche Elecsys anti-SARS-CoV-2 S assay. Quantitative antibody determination may help determine antibody titer and facilitate longitudinal monitoring of the antibody response, including antibody response to vaccines. While most serologic assays are qualitative, a quantitative serologic assay was recently developed that measures antibodies against the S protein, the target of vaccines. Molecular-based testing is used to diagnose COVID-19, and serologic testing of antibodies specific to SARS-CoV-2 is used to detect past infection.
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