In the wake of the COVID‐19 pandemic, the antibody responses to the severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) virus have received a huge interest for diagnostic or epidemiological purposes.¹ Therefore, a range of serological tests detecting specific antibodies to SARS‐CoV‐2 have emerged, but their performances depend on several factors, such as the methodology of the immunoassay, the viral antigen used for antibody binding, and the isotype detection.² In this study, the detection of total antibodies (including immunoglobulin G [IgG] and IgM) to SARS‐CoV‐2 has been performed on the Advia Centaur XP platform by using a new commercial immunoassay from Siemens Healthineers®. This chemiluminescent technique allows the detection of antibodies that recognized a recombinant receptor binding domain (RBP) protein from the coronavirus Spike protein S1. Antibodies targeting the viral RBP tend to have neutralizing capacities and to confer protective immunity.³ The Siemens SARS‐CoV‐2 assay was carried out according to the manufacturer's instructions and its recommended cutoff of 1 was applied for results interpretation (index ≥ 1 means positive while index <1 is negative).

The local ethical committee of the CHU Tivoli approved this study.

The sensitivity was determined by investigating 246 residual serums collected longitudinally over the course of time from 81 SARS‐CoV‐2‐infected patients with a positive reverse‐transcription polymerase chain reaction (RT‐PCR; or COVID‐antigen in three cases) on nasopharyngeal swab at the time of diagnosis. The performances were analyzed by receiver operating characteristic curves at different times between the PCR and blood sampling.

Since the test detects both IgG and IgM, the first‐week post‐PCR was divided into two parts to specify the sensitivity during the early phase of infection. The samples were classified into five categories: 0–2 days (n = 58), 3–6 days (n = 48), 7–13 days (n = 63), 14–20 days (n = 44), and 21–28 days (n = 33) after the positive RT‐PCR. The diagnostic sensitivity was 18.97% (95% confidence interval [CI]: 9.9%–31.4%) at 0–2 days, 52.08% (95% CI: 37.2%–66.7%) at 3–6 days, 79.37% (95% CI: 67.3%–88.5%) at 7–13 days, 90.91% (95% CI: 78.3%–97.5%) at 14–20 days, and 93.94% (95% CI: 79.8%–99.3%) at 21–28 days post‐RT‐PCR. Figure 1 shows that 4/44 patients remained negative 2 weeks after RT‐PCR. In two cases, the inability to detect anti‐SARS‐CoV‐2 persisted

On late samples taken until Days 32 and 37. For the other two, delayed samples were not available. All but one were also negative for anti‐SARS‐CoV‐2 IgG when serum samples were analyzed using another serological assay (Liaison® SARS‐CoV‐2 IgG, Diasorin® measuring anti‐S1/S2 IgG). The antibody response remains unclear for asymptomatic subjects.⁴ At 14–20 days post‐RT‐PCR, we observed a higher proportion of false‐negative among asymptomatic carriers (2/11), as compared with symptomatic patients (2/33) so that the sensitivity reached 94.29% (95% CI: 80.8%–99.3%) in this latter group.

The timeframe between the first clinical manifestations and the completion of the nasopharyngeal swab was quite variable (median: 5 days, range: 0–14 days). Therefore, for 65/70 symptomatic patients for whom the beginning of the infection was mentioned in the medical records, the sensitivity was also calculated considering the time since symptom onset. The sensitivity was 18.18% (95% CI: 8.2%–32.7%) at 0–6 days, 59.65% (95% CI: 45.8%–72.4%) at 7–13 days, 83.67% (95% CI: 70.3%–92.7%) at 14–20 days, and 100% for samples collected ≥21 days after the first symptoms. It means that all but one (for whom a follow‐up sample was not available) false‐negative patients at J14–J20 developed antibodies beyond 21 days after the first clinical manifestations.

To assess the specificity, 82 residual serum fractions collected before November 2019 were studied. It included 26 prepandemic clinical samples and 56 samples with possible confounding factors, such as Mycoplasma pneumoniae IgM (n = 15), HBsAg (n = 8), hepatitis C virus antibodies (n = 4), cytomegalovirus IgM (n = 7), EBV IgM (n = 10), toxoplasma IgM (n = 3), rheumatoid factor (n = 2), antinuclear antibodies >1/1280 (n = 4), and monoclonal immunoglobulins (n = 3). No false positive was detected and the results were clearly below the positivity threshold with a median index of 0.16 (range: <0.05–0.4). Based on Youden's index (sensitivity + specificity − 1), the specificity remained excellent up to a cutoff of 0.4. Considering this threshold, the sensitivity was 97.73% (95% CI: 88.0%–99.9%) at 14–20 days post‐RT‐PCR. However, further studies on larger cohorts are mandatory to confirm this hypothesis.

The Siemens assay automated on a Centaur XP platform appears to be a promising serological test to detect total anti‐SARS‐CoV‐2 antibodies that provides within the second week after the RT‐PCR, a sensitivity of 90.91% and even 94.29%, when only symptomatic patients are included. All controls were tested negative, leading to a specificity of 100% in our pre‐COVID cohort. Hence, this test allows reliable and rapid detection of antibodies generated secondarily to COVID‐19 infection.

在COVID-19大流行之后，针对严重急性呼吸系统综合症冠状病毒2（SARS-CoV-2）病毒的抗体反应已引起诊断或流行病学的广泛兴趣。¹因此，出现了一系列检测SARS-CoV-2特异性抗体的血清学检测方法，但其性能取决于多种因素，例如免疫测定的方法，用于抗体结合的病毒抗原和同种型检测。^2个在这项研究中，已使用SiemensHealthineers®的新型商业免疫测定法在Advia Centaur XP平台上检测了针对SARS-CoV-2的总抗体（包括免疫球蛋白G [IgG]和IgM）。这种化学发光技术可以检测从冠状病毒Spike蛋白S1中识别出重组受体结合域（RBP）蛋白的抗体。靶向病毒RBP的抗体往往具有中和能力并赋予保护性免疫力。³西门子SARS‐CoV‐2分析根据制造商的说明进行，其建议的临界值1适用于结果解释（指数≥1表示阳性，而指数<1表示阴性）。

CHU Tivoli的当地伦理委员会批准了这项研究。

通过调查在一段时间内从81例SARS‐CoV‐2感染的患者经鼻咽反转录聚合酶链反应（RT‐PCR或三例COVID抗原）阳性的246份残留血清中纵向收集的血清来确定敏感性诊断时拭子。通过在PCR和血液采样之间的不同时间的接收器操作特征曲线来分析性能。

由于测试同时检测了IgG和IgM，因此PCR后的第一周分为两部分，以指定感染早期的敏感性。样本分为五类：0至2天（n  = 58），3至6天（n  = 48），7至13天（n  = 63），14至20天（n  = 44）和21 –28天（n = 33）阳性RT-PCR之后。在0–2天时的诊断敏感性为18.97％（95％置信区间[CI]：9.9％–31.4％），在3–6天时为52.08％（95％CI：37.2％–66.7％），为79.37％（95在7-13天时的CI百分比：67.3％–88.5％；在14-20天时的90.91％（95％CI：78.3％–97.5％）和21.93％（95％CI：79.8％–99.3％） RT-PCR后– 28天。图1显示RT-PCR 2周后4/44例患者仍为阴性。在两种情况下，仍然无法检测到抗SARS-CoV-2

在第32和37天之前采集的较晚样品。对于其他两个样品，没有延迟样品。当使用另一种血清学分析法（Liaison®SARS-CoV-2 IgG，Diasorin®测量抗S1 / S2 IgG）分析血清样本时，除SARS-CoV-2 IgG以外的所有抗体均为阴性。对于无症状的受试者，抗体应答仍然不清楚。⁴在RT-PCR后14-20天，与有症状的患者（2/33）相比，我们发现无症状携带者中假阴性的比例更高（2/11），因此敏感性达到94.29％（95％） CI：后者的80.8％–99.3％）。

从最初的临床表现到完成鼻咽拭子之间的时间范围是非常可变的（中位数：5天，范围：0-14天）。因此，对于在病历中提到感染开始的65/70位有症状患者，还应考虑自症状发作以来的时间来计算敏感性。在0-6天时灵敏度为18.18％（95％CI：8.2％–32.7％），在7-13天时灵敏度为59.65％（95％CI：45.8％–72.4％），83.67％（95％CI：70.3％） –92.7％）在14–20天时；对于100％≥21天后首次出现症状的样本。这意味着J14–J20病假阴性患者中，只有一名（无随访样本）（除无随访样本外），在首次临床表现后超过21天就产生了抗体。

为了评估特异性，研究了2019年11月之前收集的82个残留血清组分。它包括26个大流行前临床样本和56个可能带有混杂因素的样本，例如肺炎支原体IgM（n  = 15），HBsAg（n  = 8），丙型肝炎病毒抗体（n  = 4），巨细胞病毒IgM（n  = 7）， EBV IgM（n  = 10），弓形虫IgM（n  = 3），类风湿因子（n  = 2），抗核抗体> 1/1280（n  = 4）和单克隆免疫球蛋白（n = 3）。未检测到假阳性，结果明显低于阳性阈值，中位指数为0.16（范围：<0.05-0.4）。根据尤登指数（灵敏度+特异性− 1），特异性一直保持在0.4的极限。考虑到该阈值，RT-PCR后14-20天的敏感性为97.73％（95％CI：88.0％-99.9％）。但是，必须对更大的人群进行进一步的研究以证实这一假设。

在Centaur XP平台上自动化的Siemens测定法似乎是一种有希望的血清学检测，可检测RT-PCR后第二周内提供的总抗SARS-CoV-2抗体，灵敏度为90.91％，甚至94.29％。仅包括有症状的患者。所有对照均检测为阴性，因此在我们COVID之前的队列中特异性为100％。因此，该测试可以可靠，快速地检测继发于COVID-19感染的抗体。