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

BMJ. 2020 Nov 11;371:m4262. doi: 10.1136/bmj.m4262.

Abstract

Objective: To assess the accuracy of the AbC-19 Rapid Test lateral flow immunoassay for the detection of previous severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

Design: Test accuracy study.

Setting: Laboratory based evaluation.

Participants: 2847 key workers (healthcare staff, fire and rescue officers, and police officers) in England in June 2020 (268 with a previous polymerase chain reaction (PCR) positive result (median 63 days previously), 2579 with unknown previous infection status); and 1995 pre-pandemic blood donors.

Main outcome measures: AbC-19 sensitivity and specificity, estimated using known negative (pre-pandemic) and known positive (PCR confirmed) samples as reference standards and secondly using the Roche Elecsys anti-nucleoprotein assay, a highly sensitive laboratory immunoassay, as a reference standard in samples from key workers.

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% confidence interval 88.8% to 95.1%) and specificity was 97.9% (97.2% to 98.4%). Using an immunoassay reference standard, sensitivity was 94.2% (90.7% to 96.5%) among PCR confirmed cases but 84.7% (80.6% to 88.1%) among other people with antibodies. This is consistent with AbC-19 being more sensitive when antibody concentrations are higher, as people with PCR confirmation tended to have more severe disease whereas only 62% (218/354) of seropositive participants had had symptoms. If 1 million key workers were tested with AbC-19 and 10% had actually been previously infected, 84 700 true positive and 18 900 false positive results would be projected. The probability that a positive result was correct would be 81.7% (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 overestimation of assay performance in studies involving only PCR confirmed cases, owing to "spectrum bias." Assuming that 10% of the tested population have had SARS-CoV-2 infection, around one in five key workers testing positive with AbC-19 would be false positives.

Study registration: ISRCTN 56609224.

Publication types

  • Observational Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Betacoronavirus
  • COVID-19
  • COVID-19 Testing
  • Clinical Laboratory Techniques / standards*
  • Coronavirus Infections / diagnosis*
  • Female
  • Firefighters
  • Health Personnel
  • Humans
  • Immunoassay / standards*
  • Male
  • Pandemics
  • Pneumonia, Viral / diagnosis*
  • Police
  • Predictive Value of Tests
  • Reagent Kits, Diagnostic / standards
  • SARS-CoV-2
  • Sensitivity and Specificity
  • United Kingdom

Substances

  • Reagent Kits, Diagnostic