Accuracy of the UK-Rapid-Test Consortium (UK-RTC) “AbC-19TM Rapid Test” for the detection of previous SARS-CoV-2 infection in key workers: test accuracy study

Objective: To assess accuracy of the AbC-19 Rapid Test lateral flow immunoassay for the detection of prior SARS-CoV-2 infection.
Design: Test accuracy study.
Setting: Laboratory based evaluation.
Participants: 2,847 key workers (healthcare staff, Fire & Rescue and Police officers) in England in June 2020: 268 with a previous PCR positive result (median 63 days previously), 2,579 with unknown previous infection status; 1,995 pre-pandemic blood donors.
Main outcome measures: AbC-19 sensitivity and specificity, estimated using known negative (pre-pandemic) and known positive (from PCR-confirmed individuals) samples as reference standards; using the Roche Elecsys anti-Nucleoprotein assay, a highly sensitive laboratory immunoassay, as a reference standard in keyworker samples.
Results: Test result bands were often weak, with positive/negative discordance by three trained laboratory staff for 3.9% of devices. Using consensus readings, for known positive and negative samples sensitivity was 92.5% (95% CI 88.8% to 95.1%) and specificity 97.9% (95% CI 97.2% to 98.4%). Using an immunoassay reference standard, sensitivity was 94.2% (95% CI 90.7% to 96.5%) among PCR-confirmed cases but only 84.7% (95% CI 80.6% to 88.1%) among other individuals with antibodies, consistent with AbC-19 being more sensitive when antibody levels are higher, as PCR confirmed individuals tended to have more severe disease whereas only 62% of seropositive individuals had been symptomatic. If 1 million keyworkers were tested with AbC-19, and 10% had actually been previously infected, we projected 84,700 true positive and 18,900 false positive results. The probability a positive result was correct would be 81.7% (95% CI 76.8% to 85.8%).
Conclusions: AbC-19 sensitivity was lower among unselected populations than among PCR confirmed cases of SARS-CoV-2, highlighting the scope for over-estimation of assay performance in studies involving only PCR-confirmed cases, due to “spectrum bias”. Assuming 10% of the tested population have had SARS-CoV-2, around one in five key workers testing positive with AbC-19 would be false positives.