The role of serologic testing for SARS-CoV-2, both in the clinical and public health settings, will continue to evolve as we gain increasing insight into our immune response to the virus. Here, we evaluated four high throughput serologic tests for detection of anti-SARS-CoV-2 IgG antibodies, including assays from Abbott Laboratories (Abbott Park, IL), Epitope Diagnostics Inc. (San Diego, CA), Euroimmun (Lubeck, Germany), and Ortho-Clinical Diagnostics (Rochester, NY), using a panel of serially collected serum samples (N=224) from 56 patients with confirmed COVID-19, healthy donor sera from 2018 and a cross-reactivity serum panel collected in early 2020. Sensitivity of the Abbott, Epitope, Euroimmun and Ortho-Clinical IgG assays in convalescent serum samples collected more than 14 days post symptom onset or initial positive RT-PCR result was 92.9% (78/84), 88.1% (74/84), 97.6% (82/84) and 98.8% (83/84), respectively. Among unique convalescent patients, sensitivity of the Abbott, Epitope, Euroimmun and Ortho-Clinical anti-SARS-CoV-2 IgG assays was 97.3% (36/37), 73% (27/37), 94.6% (35/37) and 97.3% (36/37), respectively. Overall assay specificity and positive predictive values based on a 5% prevalence rate are 99.6%/92.8%, 99.6%/90.6%, 98.0%/71.2% and 99.6%/92.5%, respectively, for the Abbott, Epitope, Euroimmun and Ortho-Clinical IgG assays. In conclusion, we show high sensitivity in convalescent sera and high specificity for the Abbott, Euroimmun and Ortho-Clinical anti-SARS-CoV-2 IgG assays. With the unprecedented influx of commercially available serologic tests for detection of antibodies against SARS-CoV-2, it remains imperative that laboratories thoroughly evaluate such assays for accuracy prior to implementation.

Serologic evaluation for Zika virus (ZIKV) infection currently includes an initial screen using an anti-ZIKV IgM antibody capture enzyme-linked immunosorbent assay (MAC-ELISA) followed by supplemental testing of specimens with nonnegative results by a plaque reduction neutralization test (PRNT). We compared the performance characteristics of three ELISAs for the detection of IgM class antibodies to ZIKV, including the Centers for Disease Control and Prevention (CDC) Zika MAC-ELISA, the InBios ZIKV Detect MAC-ELISA, and the Euroimmun anti-Zika Virus IgM ELISA. Additionally, we present our initial experiences with ZIKV serologic testing from a national reference laboratory perspective. Using both retrospectively and prospectively collected specimens from patients with possible ZIKV infection, we show that the CDC and InBios MAC-ELISAs perform comparably to each other, with positive agreement, negative agreement, and interrater kappa values ranging from 87.5% to 93.1%, 95.7% to 98.5%, and 0.52 to 0.83, respectively. In contrast, comparison of the Euroimmun ZIKV ELISA to either the CDC or InBios MAC-ELISAs resulted in positive agreement, negative agreement, and interrater kappa values ranging from 17.9% to 42.9%, 91.7% to 98.6%, and 0.10 to 0.39, respectively. Among the 19 prospective samples submitted for PRNT, nine were negative, eight specimens had neutralizing antibodies to a flavivirus (unable to be identified), and one sample each was confirmed for ZIKV or dengue virus infection. This study highlights the ongoing challenges associated with serologic diagnosis of ZIKV infection. Although the availability of a commercial serologic test for ZIKV has greatly expanded the national capacity for such testing, the need to further characterize and improve these assays, particularly with regard to specificity, remains. KEYWORDS Zika virus, serologyZ ika virus (ZIKV) emerged from obscurity in early 2015 following its detection in Bahia, Brazil (1). Over the next year, ZIKV spread rapidly throughout Latin America, the Caribbean, and into the southern United States, resulting in a major and still ongoing international outbreak (2). Currently, over 1 million cases of suspected or confirmed ZIKV infection have been documented in the Americas by the Pan American Health Organization (3). ZIKV, a single-stranded RNA virus and member of the Flavivirus genus, is primarily transmitted through infected Aedes species mosquitoes, which are also primary vectors for dengue (DENV) and chikungunya (CHIKV) viruses, both of which cocirculate in many regions where ZIKV is now considered endemic (4). ZIKV transmis-

The QuantiFERON-TB Gold Plus (QFT-Plus; Qiagen, Germantown, MD) interferon gamma release assay (IGRA) received FDA clearance in 2017 and will replace the prior version of the assay, the QFT-Gold In-Tube (QFT-GIT). Here, we compared performances of the QFT-Plus assay and the QFT-GIT version in a diverse patient population, including patients undergoing evaluation for or follow-up of latent tuberculosis infection (LTBI; = 39) or active TB infection ( = 3), and in health care workers (HCWs; = 119) at Mayo Clinic (Rochester, MN). Compared to the QFT-GIT, the QFT-Plus assay showed 91.2% (31/34) positive, 98.4% (124/126) negative, and 96.6% (156/161) overall qualitative agreement among the 161 enrolled subjects, with a Cohen's kappa value of 0.91 (excellent interrater agreement). Among the 28 patients diagnosed with LTBI at the time of enrollment, the QFT-GIT and QFT-Plus assays agreed in 24 (85.7%) patients; in all four discordant patients, the positivity of the QFT-GIT or QFT-Plus IGRA was associated with low-level interferon gamma (IFN-γ) reactivity, ranging from 0.36 IU/ml to 0.66 IU/ml. Additionally, we document a high degree of correlation between IFN-γ levels in the QFT-GIT TB antigen tube and each of the two QFT-Plus TB antigen tubes, as well as between the QFT-Plus TB1 and TB2 tubes (Pearson's correlation coefficients [] > 0.95). Overall, we show comparable results between the QFT-GIT and QFT-Plus assays in our study population composed of subjects presenting with a diverse spectrum of TB infections. Our findings suggest that the necessary transition to the QFT-Plus assay will be associated with a minimal difference in assay performance characteristics.

The COVID-19 pandemic led to development of numerous serologic tests. To obviate the need for phlebotomy services, we validated dried blood spots (DBS) for anti-SARS-CoV-2 serologic testing. Immunoglobulins were extracted from 3 mm DBS punches and the extracts were analyzed using the Euroimmun anti-SARS-CoV-2 IgG ELISA. Various pre-analytical factors were studied. Results were favorable for DBS stored for at least 67 days at or below 37°C. Comparison between paired serum and DBS specimens tested by the Euroimmun ELISA showed 96.8% and 81.3% positive and negative agreement, respectively, indicating that confirmatory testing of positive Euroimmun results on DBS extracts is necessary to achieve clinical accuracy. Our findings suggest that any SARS-CoV-2 antibody assay that requires pre-dilution of serum is amenable to DBS as an alternate specimen type that is relatively low cost, self-collectable, stable, can be shipped by standard mail and could be used to establish the seroprevalence of large populations.

Background Dried blood spots (DBS) are an established specimen type for clinical testing given their low cost, ease of collection and storage, and convenient shipping capabilities through the postal system. These attributes are complementary to the expansion of SARS-CoV-2 serologic testing, which may be used to inform community seroprevalence rates. Methods The Luminex xMAP SARS-CoV-2 Multi-Antigen assay utilizes magnetic beads labeled with three viral antigens (nucleocapsid [NC], receptor binding domain [RBD], spike S1 subunit) to detect anti-viral IgG-class antibodies, and has Food and Drug Administration (FDA) Emergency Use Authorization (EUA) for use in serum and plasma. This assay was modified for use with DBS and validated against paired sera tested by one of two reference assays: the Roche Diagnostics Elecsys anti-SARS-CoV-2 ECLIA or the Euroimmun anti-SARS-CoV-2 IgG ELISA. Results 159 paired DBS and serum specimens analyzed using the modified Luminex xMAP assay on DBS and the reference methods on serum showed an overall concordance of 96.9% (154/159). Use of multivariate pattern recognition software (CLIR) for post-analytical interpretation of the Luminex xMAP DBS assay results, instead of manufacturer provided interpretive thresholds, increased overall qualitative result concordance to 99.4% (158/159) between the modified Luminex xMAP DBS and reference results. Conclusions Use of DBS for detection of antibodies against SARS-CoV-2 provides comparable results to those obtained using serum. DBS concordance was improved with multivariate pattern recognition software (CLIR). We demonstrate that DBS are a reliable specimen type for SARS-CoV-2 antibody detection using the modified Luminex xMAP assay.

Objective To estimate the seroprevalence of SARS-CoV-2 antibodies in healthcare personnel. Methods The Mayo Clinic Serology Screening Program was created to provide a voluntary, two-stage testing program for SARS-CoV-2 antibodies to healthcare personnel. The first stage utilized a dried blood spot screening test initiated on June 15, 2020. Those participants identified as reactive were advised to have confirmatory testing via a venipuncture. Venipuncture results through August 8, 2020 were considered. Consent and authorization for testing was required in order to participate in the screening program. This report, which was conducted under an institutional review board approved protocol, only includes employees who have further authorized their records for use in research. Results A total of 81,113 healthcare personnel were eligible for the program, and of these 29,606 participated in the screening program. A total of 4,284 (14.5%) of the dried blood spot test results were “reactive” and warranted confirmatory testing. Confirmatory testing was completed on 4,094 (96%) of the screen reactive with an overall seroprevalence rate of 0.60% (95% CI: 0.52% to 0.69%). Significant variation in seroprevalence was observed by region of the country and age group. Conclusions The seroprevalence for SARS-CoV-2 antibodies through August 8, 2020 was found to be lower than previously reported in other health care organizations. There was an observation that seroprevalence may be associated with community disease burden.

Background Toxoplasmosis is routinely diagnosed through detection of Toxoplasma gondii–specific antibodies. However, the imperfect specificity of T. gondii serologic assays is a well-recognized limitation. The new BioPlex 2200 (Bio-Rad Laboratories) T. gondii, rubella, and cytomegalovirus (ToRC) IgM multiplex flow immunoassay (MFI) received FDA clearance in May 2017. We evaluated the clinical performance of the new T. gondii IgM and existing IgG portion of this MFI. Methods Three hundred prospectively collected consecutive, residual sera, submitted for T. gondii serologic testing as part of routine clinical care, and an archived set of 52 residual sera previously positive for anti–T. gondii IgM and IgG with the predicate ADVIA Centaur Toxoplasma IgM and IgG assays (Siemens) were evaluated. Performance of the BioPlex 2200 T. gondii IgM and IgG MFIs was assessed by calculating positive (PPA) and negative percent agreement (NPA) compared to the Centaur tests. Results Among prospective specimens, the BioPlex 2200 T. gondii IgM and IgG MFIs demonstrated a PPA of 0% (0/7) and 82.3% (28/34) and NPA of 99.3% (288/290) and 95.8% (251/262), respectively, with the Centaur assays. Chart review of the 7 Centaur T. gondii IgM-positive samples revealed that these were likely falsely positive. Among archived samples, the BioPlex 2200 T. gondii IgM and IgG MFIs showed PPAs of 90.4% (47/52) and 100% (52/52), respectively. Conclusions The BioPlex 2200 T. gondii IgM and IgG MFIs demonstrated excellent concordance with the Centaur assays. The T. gondii IgM MFI may provide higher specificity in low-prevalence populations.

TB remains a leading cause of morbidity and mortality worldwide. However, most infected immunocompetent individuals are asymptomatic and only 5–10% of these will eventually develop active TB during their lifetime (typically within 2 years after exposure). Therefore, rapid diagnosis and efficient management of asymptomatic infected individuals who are at the highest risk of progression and transmission remain major clinical and public health challenges. In recent years, there has been important scientific progress in our understanding of the spectrum of asymptomatic Mycobacterium tuberculosis (Mtb) infections that not only includes the dynamic state of latent TB infection (LTBI), but also the preclinical state of incipient and subclinical TB. The latter is possibly as prevalent as symptomatically active TB and potentially contributes to global Mtb transmission in various settings. We summarize the latest developments and current challenges of the existing testing tools for LTBI and describe promising biomarkers and diagnostics for the spectrum of asymptomatic TB. Following the negative results of a recent clinical trial for a biomarker-guided preventive therapy approach, we also suggest some treatment options for incipient TB.