Evaluation of self-collected dried blood spots for detection of SARS-CoV-2 nucleocapsid antibodies shows low sensitivity.

Background And Aims: Dried blood spots (DBS) have been proposed as a cost-effective surveillance method for population-wide screening of SARS-CoV-2 immunity but sensitivity of DBS based on self-collected DBS samples is unknown. To evaluate the success of vaccination strategies, it is necessary to differentiate vaccination from natural infection. Therefore, a test for antibodies against the viral nucleocapsid protein (anti-N) is desirable.

Materials And Methods: In our prospectively followed cohort of healthcare workers (HCW) in eastern Switzerland, we assessed SARS-CoV-2-anti-N-seroprevalence using DBS on a biweekly basis from March to September 2020. Phlebotomy samples were collected in March and September and tested for anti-N-seropositivity, as well as SARS-CoV-2 spike antibodies for quantitative validation. Venous antibody testing was compared with DBS results for anti-N using the Roche Elecsys electro-chemiluminescence immunoassay.

Results: 792 HCW (median age 38.3 years) were included, 35 (4.4 %) were SARS-CoV-2-anti-N-seropositive. Of 43 matching DBS, 25 tested positive for anti-N, accounting for a sensitivity of 58.1 % (95 %CI 43.3-71.6 %). We found a significant correlation of anti-N from DBS with results from phlebotomy samples (r = 0.77;p < 0.0001), with higher levels being associated with a higher true-positive rate. Anti-N in DBS correlated significantly with quantitatively validated anti-S obtained from serum (r = 0.67;p < 0.0001).

Conclusion: Although home DBS collection was feasible in a larger cohort and we found a high correlation between anti-N detection in DBS and phlebotomy samples, the sensitivity of self-collected DBS samples was significantly impaired for the Roche Elecsys anti-N assay. Therefore, we cannot recommend this method for DBS when testing from venous blood is possible.