Using Prenatal Blood Samples to Evaluate COVID-19 Rapid Serologic Tests Specificity

INTRODUCTION: Background cross-reactivity with other coronaviruses may reduce the specificity of COVID-19 rapid serologic tests. The vast majority of women attend prenatal care, which is a unique source of population-based blood samples appropriate for validation studies. We used stored 2018 serum samples from an existing pregnancy cohort study to evaluate the specificity of COVID-19 serologic rapid diagnostic tests. METHODS: We randomly selected 120 stored serum samples from pregnant women enrolled in a cohort in 2018 in Tegucigalpa, Honduras, at least 1 year before the COVID-19 pandemic. We used stored serum to evaluate four lateral flow rapid diagnostic tests, following manufacturers’ instructions. Pictures were taken for all tests and read by two blinded trained evaluators. RESULTS: We evaluated 120, 80, 90, and 90 samples, respectively. Specificity for both IgM and IgG was 100% for the first two tests (95% confidence intervals [CI] 97.0–100 and 95.5–100, respectively). The third test had a specificity of 98.9% (95% CI 94.0–100) for IgM and 94.4% (95% CI 87.5–98.2) for IgG. The fourth test had a specificity of 88.9% (95% CI 80.5–94.5) for IgM and 100% (95% CI 96.0–100) for IgG. DISCUSSION: COVID-19 serologic rapid tests are of variable specificity. Blood specimens from sentinel prenatal clinics provide an opportunity to validate serologic tests with population-based samples.