Diagnostic accuracy of the ultrasensitive S-PLEX SARS-CoV-2 N electrochemiluminescence immunoassay

We read with interest the recent article of Ren et al. [1], who described the accuracy of an ultrasensitive electrochemiluminescence immunoassay for saliva-based Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) nucleocapsid protein (N) detection based on the S-PLEX platform (S-PLEX SARS-CoV-2 N Kit; Meso Scale Discovery, Rockville, MD, United States). This method has been specifically developed for detecting and quantifying the SARS-CoV-2 N antigen in a variety of human specimens, including serum, plasma, saliva and nasopharyngeal swabs (NPS). Briefly, either S-PLEX 96-Well SECTOR or QuickPlex plates coated with streptavidin for binding biotin-conjugated capture anti-SARS-CoV-2 N antibodies are challenged with human samples. After this step, “TURBO-BOOST”-labeled detection antibodies react with the N antigen bound to the solid phase and, after addiction of a specific reagent, an electrochemiluminescent signal is generated and read by the specific instrument. The signal produced is proportional to the concentration of N antigen present in the test sample. A preliminary evaluation of this assay revealed that the limit of detection is 0.16 pg/mL, with a diagnostic threshold set at 0.32 pg/mL and a total imprecision ranging between 7.0 and 7.7% [2]. The sample volume is only 25 μL, with total turnaround time between 4–5 h. Since this novel technique displayed remarkable diagnostic performance in saliva samples in the hands of Ren and colleagues, exhibiting up to 100% specificity with 92% sensitivity [1], we provide here a critical literature review and pooled analysis of studies which addressed the accuracy of S-PLEX SARS-CoV-2 N Kit for diagnosing acute SARS-CoV-2 infections. We carried out a digital search in the two scientific databases Medline (PubMed interface) and Scopus, using the following keywords: “S-PLEX” AND “COVID-19” OR “SARS-CoV-2”, without no language or date (i.e., up to February 17, 2022) restrictions. The initial screening of documents was conducted by G.L. and M.M., aimed at selecting studies were the diagnostic accuracy of S-PLEX SARS-CoV-2 N Kit was assessed against a reference molecular technique for diagnosing acute SARS-CoV-2 infections, and with sufficient extrapolable information for construction of a 2×2 table. A pooled analysis, based on the Mantel-Haenszel method and random effects model, was employed for estimating the diagnostic sensitivity, specificity and accuracy (reported as Summary Receiver Operating Characteristic Curve [SROC] and agreement) of this method. The inter-study heterogeneity was also assessed with χ test and I statistic. The statistical analysis was performed with Meta-DiSc 1.4 (Unit of Clinical Biostatistics team of the Ramón y Cajal Hospital, Madrid, Spain) [3]. The analysis was carried out in accordance with the Declaration of Helsinki and within the terms of Martina Montagnana and Mario Plebani share senior authorship of this work.