RT-PCR tests to detect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA are the operational gold standard for detecting COVID-19 disease in clinical practice. RT-PCR assays in the UK have analytical sensitivity and specificity of greater than 95%, but no single gold standard assay exists. New assays are verified across panels of material, confirmed as COVID-19 by multiple testing with other assays, together with a consistent clinical and radiological picture. These new assays are often tested under idealised conditions with hospital samples containing higher viral loads than those from asymptomatic individuals living in the community. As such, diagnostic or operational performance of swab tests in the real world might differ substantially from the analytical sensitivity and specificity.
A false positive is when someone who does not have coronavirus, tests positive for it.
No test is 100% accurate - there will always be some people who test positive when they do not have the disease, or test negative when they do have it.
False positives in any testing programme are important - especially when there is low prevalence of a disease - because they could potentially make us think there are significantly more cases of something than there really are.
The false positive rate usually refers to the number of people who are not infected but get positive results, as a proportion of all the people tested who really don't have the virus. We do not know what the precise rate is though.
Dr Paul Birrell, a statistician at the Medical Research Council's Biostatistics Unit at the University of Cambridge, says: "The false positive rate is not well understood and could potentially vary according to where and why the test is being taken. A figure of 0.5% for the false positive rate is often assumed."
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