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The UK government guidance on diagnosis is based on clinical symptoms. Testing (not specified) is recommended for cases who are well enough to remain in the community. No guidance is given as to how to interpret such a test or any actions that should be taken consequent to the test results. Thus, new cases in the UK could reasonably be thought to mean cases diagnosed by clinical symptoms.
Public Health England describes the four pillars of testing to include swab testing and additionally serology testing for certain groups. The methodology for counting cases states the following:
“If a person has both a negative and a positive test, then only their positive test will be counted. If a person is tested as positive under both pillar 1 and pillar 2, then only the first positive case is counted.”
An asymptomatic person who tested positive could have two confirmatory negative tests, but would still count as a confirmed case. In Wales, data is deduplicated on 42-day episodes; if someone is tested twice, 43 days apart, they will be included in the case count measure twice.
The UK government updates its guidance and recently posted on assurances of positive results during periods of low prevalence.
The latest guidance states that ‘positive test results at the limit of detection that occur early in the cycle of infection are important as these represent individuals who may go on to transmit infection.’ The guidance asks laboratories to ‘determine the threshold for a positive result at the limit of detection based on the in-use assay,’ without stating what the threshold should be. If necessary, the laboratory should request a repeat sample; again this advice is given without a threshold to guide when to do the repeat test.
What is the case definition being used for clusters of UK cases being reported currently?
We deduce that a reported “case” is most probably simply the result of a positive PCR test. The new guidance is meaningless unless it provides a clear threshold for the limits of detection. For many whose test turns up positive, there may be nothing recorded about any clinical symptoms.