The accuracy of the rapid finger-prick antibody test for SARS-CoV-2 may be considerably lower than previously suggested, finds a study published by the BMJ.
The study assessed the accuracy of the AbC-19 Rapid Test lateral flow immunoassay for the detection of previous SARSCoV-2 infection in 2847 key workers (health care staff, fire and rescue officers and police officers) in England in June 2020. Of these, 268 had had a previous polymerase chain reaction (PCR) positive result (median, 63 days previously). There were 2579 with unknown previous infection status and 1995 pre-pandemic blood donors.
AbC-19 sensitivity and specificity were estimated using known negative (pre-pandemic) and known positive (PCR confirmed) samples as reference standards.
The findings showed that test result bands were often weak, with positive/negative discordance by three trained laboratory staff for 3.9 per cent of devices. Using consensus readings, for known positive and negative samples, sensitivity was 92.5 per cent (95% CI, 88.8-95.1%) and specificity was 97.9 per cent (95% CI, 97.2-98.4%).
Using an immunoassay reference standard, sensitivity was 94.2 per cent (95% CI, 90.7-96.5%) among PCR-confirmed cases but was 84.7 per cent (95% CI, 80.6-88.1%) among others 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 per cent of seropositive participants had had symptoms.
Assuming that 10 per cent of the tested population had been infected with SARSCoV-2, around one in five key workers testing positive with AbC-19 would be false positives. If 1 million key workers were tested with AbC-19 and 10 per cent 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 per cent (95% CI, 76.8-85.8%).
