Diagnostic accuracy for a plasma SARS-CoV-2 Nucleocapsid Protein method [version 1; peer review: 1 approved with reservations]

Background: The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) releases nucleocapsid proteins (NP) into the blood circulation in infected patients. We investigated whether plasma NP analysis could be used for diagnosing an infection and used for nosocomial screening. Methods: We collected blood samples from patients admitted to the hospital during a period with reverse transcription polymerase chain reaction (RT-PCR) based-screening of patients for SARS-CoV-2. Retrospectively the SARS-CoV-2 NP plasma concentrations were measured with an enzyme-linked immunosorbent assay (ELISA) method and used for an initial time course study to find the optimal time-point for sampling blood. Next, we estimated the diagnostic accuracy i.e. the clinical sensitivity and specificity at different plasma NP cut-off concentrations. Results: The time course study revealed profiles with rapid or more slow declines in NP titers after the RT-PCR result. Nevertheless, in the time interval 0 - 7 days after the RT-PCR result, the NP concentration was always above the level of detection at 1.66 pg/ml suggesting that the diagnosis could be established in the time interval of 0 - 7 days. The median time gap between the plasma NP and RT-PCR results was 0.0 days (n = 1957, interval: -26 to + 21 days). Reducing the time gap to seven days, the clinical sensitivity was 90.0% (n= 60, 95% CI, 82.4% to 97.6%) at a specificity of 95.9% (n=1876, 95% CI, 95.0% to 96.8%). Curve analysis by receiver operation characteristics identified a cut-off concentration of 1.87 pg/mL NP as optimal resulting in a positive predictive value of 41.2%, a negative predictive value of 99.7% and a prevalence of 3.1%. Conclusions: In conclusion, the NP method is acceptable for making the laboratory diagnosis of SARS-CoV-2, and an intended use of plasma NP as a prospective nosocomial screening method is considered feasible.