We next used our SARS-CoV-2 IgG index values over 415 unique patient-days to assess the change in index value over time, from the date of symptom onset ( Figure 1C) and first positive PCR result ( Figure 1D). Intriguingly, 22 of 88 individuals (25%) for which serum was available on the first day of PCR positivity had simultaneous detection of serum anti-SARS-CoV-2 IgG and nasopharyngeal SARS-CoV-2 RNA ( Figure 1B). The sensitivity of the assay from the estimated day of symptom onset for the 125 patients included in our chart-review study was 53.1% (95%CI 39.4%-66.3%) at 7 days, 82.4% (51.0-76.4%) at 10 days, 96.9% (89.5-99.5%) at 14 days, and 100% (95.1%-100%) at day 17 using the manufacturer’s recommended cutoff of 1.4. Fifty-eight percent of patients were male and 42% female. The vast majority of these patients were hospitalized at the University of Washington Medical Center-Northwest Campus in Seattle, WA between March and April 2020. To determine assay sensitivity, we used a series of 125 patients who tested RT-PCR positive for SARS-CoV-2 for which 689 excess serum specimens comprising 415 unique patient follow-up days were available. All other specimens tested negative, leading to an assay specificity of 99.90% in pre-COVID-19 serum. One serum specimen from this set tested positive with an initial index value of 1.41 and repeated at 1.49, above the Abbott-determined positivity cutoff of 1.40 ( Figure 1A). To determine assay specificity, we used 1,020 deidentified serum specimens from 1,010 different individuals sent to our laboratory for HSV Western blot serology in 20, before SARS-CoV-2 was thought to be circulating in Washington State and the United States ( 7). Sensitivity and Specificity of the Abbott SARS-CoV-2 IgG Assay This work was approved under a consent waiver by the University of Washington IRB. Serum specimens sent from the Boise, Idaho metropolitan area were collected over a one-week period in late April 2020 as part of the Crush the Curve initiative. For the calculations of sensitivity and specificity at the patient level using the manufacturer’s recommended index value cutoff of 1.40 ( Figure 1), patients were assumed to be seronegative for each day preceding the most recent negative IgG result and to be seropositive for each day following an initial positive result. With the exception of the studies of biologic precision, for patients with an IgG result on more than 1 aliquot on a specific date following onset of symptoms or PCR positivity, only the mean index value for that patient-day was included in the data set to minimize the bias from individual patient seroconversion and variable numbers of samples per patient. Sensitivity specimens were derived from excess serum specimens sent for clinical testing from persons who tested RT-PCR positive for SARS-CoV-2 during March and April 2020. Specificity samples were derived from de-identified excess serum specimens sent to our clinical virology laboratory in 20. This assay detects IgG antibodies against the SARS-CoV-2 nucleocapsid protein. Here, we evaluated the Abbott SARS-CoV-2 IgG test for use on the Abbott Architect platform. To date, most SARS-CoV-2 serological tests on the market have inadequate performance characteristics to perform widespread population or clinical testing ( 4). Serological tests require exceptional sensitivity and specificity, especially when seroprevalence is low, in order to have adequate positive predictive value ( 3). Serological testing can detect past cases of SARS-CoV-2 for which RT-PCR testing was either not performed or for which nasopharyngeal swab sampling resulted in false negatives. COVID-19 often progresses to lower respiratory tract illness and can be associated with severe morbidity and mortality ( 2). Coronavirus disease-19 (COVID19) is a novel respiratory illness caused by Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel Sarbecovirus that recently emerged from Wuhan, China in late 2019 ( 1).
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