Dried Blood Spot as an Alternative to Plasma/Serum for SARS-CoV-2 IgG Detection, an Opportunity to Be Sized to Facilitate COVID-19 Surveillance Among Schoolchildren (2020)

To the Editors: Dried blood spots (DBSs), which can be collected with minimally invasive procedures, are used for serologic investigations, such as for Measles or Rubella.1 Here, we assessed the adequacy of DBS as a tool to collect samples for the detection of SARS-CoV-2 IgG via a semiquantitative ELISA by comparing results obtained from matched serum and DBS samples from 52 Health Care Workers. Drops of capillary blood from a finger prick were collected on cellulose-based DBS cards (Perkin-Elmer 226 filter paper, Greenville, SC). Four 3-mm disks were punched out and incubated for 1 hour at room temperature in 500 µL of ELISA sample buffer with constant shaking (400 rpm) to elute antibodies from the paper. Eluates and paired sera were tested simultaneously for the presence of anti-SARS-CoV-2 IgG with a commercially available ELISA (Euroimmun Medizinische Labordiagnostika, Lubeck, Germany) according to the manufacturer’s instructions. Samples were classified according to ratio values of sample absorbance over calibrator (ODs/Cal) as recommended by the manufacturer into three categories: negative (<0.8), inconclusive (≥0.8 and <1.1), and positive (≥1.1). Pearson’s coefficient was calculated to evaluate concordance between measures and results were considered significant at P <0.05. Concordance between tests was assessed using the Kappa statistic (Cohen’s unweighted Kappa, k). Linear modeling for regression analysis was performed and visualized in R. Twenty-three serum samples resulted IgG-positive (ODs/Cal: 1.1–6.2), 28 were negative (ODs/Cal: 0.1–0.7), and 1 test was inconclusive (0.9 ODs/Cal). Similarly, 21 DBS samples were positive (ODs/Cal: 1.1–6.1), 28 negative (ODs/Cal: 0.2–0.7), and 3 tests were inconclusive (ODs/Cal: 0.8-1.0). Overall, only 2 discordant results were obtained and in both cases serum samples tested positive, while the tests performed on DBS eluates were inconclusive. The correlation coefficient (R2) between the 2 sets was 0.9553, Pearson’s coefficient was 0.98 (0.97–1, P < 0.0001), k was 0.93 (0.92–1, P < 0.0001), and the overall agreement was 96.2% (86.8–99.5). Results of the linear relation analysis are shown in Figure 1.FIGURE 1.: Linear regression analysis of results obtained with sera and DBS eluates for 52 paired samples. Also shown are observations (dots) and 95% confidence interval (gray).The analysis of both positive and negative samples demonstrated the validity of DBS as a sampling method for anti-SARS-CoV-2 IgG detection by ELISA. These data support the feasibility of the use of DBS as a simple, rapid and reliable sample collection tool for the detection of antibodies against SARS-CoV-2.2 DBS combine the convenience of noninvasive sample collection in nonclinical settings3 and room-temperature storage with the accuracy of the ELISA in the laboratory. DBSs are a valid alternative to serum/plasma for field studies. We are using this approach for monitoring the spread of SARS-CoV-2 among children upon their return to schools in Milan, one of the areas most affected by COVID-19 during the beginning of the pandemic. This will minimize the risk of measures that require drastic responses, such as further prolonged school closures. The prompt knowledge of the local epidemiologic profile will facilitate the development of targeted solutions to contrast and contain the negative effects of any epidemic ups and downs. ACKNOWLEDGMENTS The authors would like to acknowledge Joost Verhoeven for his appreciated help with statistical analysis. Antonella Amendola, PhDDepartment of Biomedical Sciences for Health, Coordinated Research Center “EpiSoMI”, University of Milan, Milan, ItalySilvia Bianchi, PhDMaria Gori, PhDDepartment of Biomedical Sciences for Health, University of Milan, Milan, ItalyLucia Barcellini, MDDepartment of Paediatrics, Children Hospital V. Buzzi, University of Milan, Milan, ItalyDaniela Colzani, BSDepartment of Biomedical Sciences for Health, University of Milan, Milan, ItalyMarta Canuti, PhDDepartment of Biology, Memorial University of Newfoundland, NL, CanadaVania Giacomet, MDValentina Fabiano, MDLaura Folgori, MDGian Vincenzo Zuccotti, MDDepartment of Paediatrics, Children Hospital V. Buzzi, University of Milan, Milan, ItalyElisabetta Tanzi, PhDDepartment of Biomedical Sciences for Health, Coordinated Research Center “EpiSoMI”, University of Milan, Milan, Italy