What causes false positives?
• Cross reactions with other genetic material. Other sources of DNA or RNA may have cross
reactive genetic material that can be amplified by the RT-PCR test. False positives were
observed unexpectedly in norovirus assays in patients with enterocolitis, due to unusually high
levels of human DNA in samples [1]
• Contamination during sampling. This may happen if the swab head accidently contacts, or is
placed on a contaminated surface (e.g. latex gloves, hospital surface).
• Contamination during swab extraction. Viral RNA is extracted from swabs in solution;
accidental aerosolization of liquid can cause cross contamination between samples.
• Contamination with PCR amplicon. The PCR amplification process generates millions of copies
of the DNA target (amplicon) that can cause false positives in subsequent PCR reactions. If a
testing lab is accidently contaminated with amplicon it can lead to sporadic false positives.
• Contamination of PCR laboratory consumables. Contamination can spread from a post-PCR lab
into a pre-PCR lab by transfer of equipment, chemicals, people or aerosol. Even experienced
national labs can be affected. In early-March 2020, COVID-19 RT-PCR assays produced by the
CDC were withdrawn after many showed false positives due to contaminated reagents. [2]
What is the UK operational false positive rate?
The UK operational false positive rate is unknown. There are no published studies on the
operational false positive rate of any national COVID-19 testing programme.
An 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 2004-2019 [7]. Results of 43 EQAs were examined,
giving a median false positive rate of 2.3% (interquartile range 0.8-4.0%).
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