Accuracy of Rapid SARS-CoV-2 Antibody Test Kits Is Limited

Several manufacturers have developed lateral flow immunoassays (LFIAs) — small, pregnancy-test-style kits for the detection of antibodies against SARS-CoV-2. Evaluations of the currently available LFIAs have typically used prepandemic blood samples as negative controls and convalescent samples from patients with polymerase chain reaction (PCR)–confirmed SARS-CoV-2 infection (COVID-19) as positive controls — likely overestimating sensitivities and specificities, as patients with severe COVID-19 are more likely to receive a diagnostic PCR and are known to develop higher antibody titers (“spectrum bias”).

British researchers evaluated a SARS-CoV-2 rapid antibody test (AbC-19) for which sensitivity and specificity of 97.7% and 100% had been claimed using an evaluation setup as described above. Testing of 1995 prepandemic serum samples obtained from blood donors in 2016 and 2017 found a 97.9% specificity for the AbC-19 test (i.e., 2.1% false positives), and in a cohort of 268 U.K. “key workers” (healthcare workers, police officers, or firefighters) with a previous positive PCR test, sensitivity of the AbC-19 test was 92.5%. Using a commercial immunoassay as the gold standard, sensitivity was 84.7% in 2579 key workers who did not know their SARS-CoV-2 status and 86.6% among all key workers irrespective of PCR-status. Based on these data, and assuming a 10% prevalence of previous COVID-19 infection, the positive predictive value of the AbC-19 test would be 82%, meaning that only 4 out of 5 people who recovered from COVID-19 would be correctly classified.