![]() ![]() There are no published studies on the operational false positive rate of any national COVID-19 testing programme.Īn attempt has been made to estimate the likely false-positive rate of national COVID-19 testing programmes by examining data from published external quality assessments ( EQAs) for RT-PCR assays for other RNA viruses carried out between 2004 to 2019. The UK operational false positive rate is unknown. What is the UK operational false positive rate? In early-March 2020, COVID-19 RT-PCR assays produced by the CDC were withdrawn after many showed false positives due to contaminated reagents. Even experienced national labs can be affected. Contamination can spread from a post- PCR lab into a pre- PCR lab by transfer of equipment, chemicals, people or aerosol. If a testing lab is accidently contaminated with amplicon it can lead to sporadic false positives.Ĭontamination of PCR laboratory consumables. The PCR amplification process generates millions of copies of the DNA target (amplicon) that can cause false positives in subsequent PCR reactions. Viral RNA is extracted from swabs in solution accidental aerosolization of liquid can cause cross contamination between samples.Ĭontamination with PCR amplicon. This may happen if the swab head accidently contacts, or is placed on a contaminated surface (for example, latex gloves, hospital surface).Ĭontamination during swab extraction. False positives were observed unexpectedly in norovirus assays in patients with enterocolitis, due to unusually high levels of human DNA in samples. Other sources of DNA or RNA may have cross reactive genetic material that can be amplified by the RT-PCR test. What causes false positives?Ĭross reactions with other genetic material. This short paper suggests this must be measured as a priority, and makes recommendations on managing operational false positive and false negative rates. We have been unable to find any data on the operational false positive and false negative rates in the UK COVID-19 RT-PCR testing programme. They will affect national surveillance, and the functioning of the UK track-and-trace programme. ![]() Operational false-positives and false-negatives will have significant impact in the way we respond to the COVID-19 pandemic. The diagnostic sensitivity and diagnostic specificity of a test can only be measured in operational conditions. In a clinical or community setting there may be inefficient sampling, lab contamination, sample degradation or other sources of error that will lead to increased numbers of false positives or false negatives. It is important to remember that laboratory testing verifies the analytical sensitivity and analytical specificity of the RT-PCR tests. This means that under laboratory conditions, these RT-PCR tests should never show more than 5% false positives or 5% false negatives. The RT-PCR assays used for the UK’s COVID-19 testing programme have been verified by PHE, and show over 95% sensitivity and specificity. RT-PCR tests are highly sensitive, but can show false negatives (giving a negative result for a person infected with COVID-19) and false positives (giving a positive result for a person not infected with COVID-19). These RT-PCR tests are carried out across a network of government, commercial and academic labs across the UK to meet the high demand and fast turnaround required. ![]() Pillar 1 (those with a clinical need, critical essential workers in the NHS) pillar 2 (essential workers, wider public through NHS portal, care home staff and residents regardless of symptoms) and pillar 4 (national surveillance, such as ONS surveys) of the UK testing programme use reverse-transcription polymerase chain reaction ( RT-PCR) tests to detect viral RNA. The UK’s COVID-19 testing programme uses a network of laboratories to detect SARS-CoV-2 in nasopharyngeal swabs. ![]()
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