Any standard for number of cycles on PCR tests?

[Julian]

I'm planning a trip to a country that requires a negative clinic-administered PCR test taken within 72 hours of departure. This will be the first trip I have taken since this pandemic hit and the only thing that is making me nervous is the possibility of testing positive just before my flight. Despite being fully vaccinated and boosted, and I think having had Covid-19 once or maybe even twice both with no severe illness, I am still a bit nervous about testing positive based on stuff I have read about how sometimes PCR tests can still detect viral fragments months after the infection has been cleared.

It is what it is and I'll just have to hope for the best but it did get me thinking about whether there are any UK government guidelines, or even regulations, regarding the number of cycles (Ct is the term I believe) that a test lab should use for its testing. Does anyone know?

If there are no definitive regulations maybe no reputable company would want to lead with this information but for my particular circumstances I would certainly be tempted to shop around for a test centre that was using a lower Ct value. I wouldn't go as far as using some company that was flagrantly flouting any lack of regulation by using a Ct value so low as to be effectively faking the testing by almost guaranteeing a negative result but if there was some reasonable range of Ct values considered by experts to be an acceptable range I would certainly want to be fishing at the lower "sensitive enough" end of the Ct value range rather than the higher "getting close to being overly sensitive" end of the range.

- Julian

[Midsmartin]

I think that there are multiple tests in use that may have different guidance from their manufacturers. Someone else might know more.

[redsturgeon]

40 cycles is the norm.

More info

https://assets.publishing.service.gov.u ... T-PCR_.pdf

John

[redsturgeon]

If there are no definitive regulations maybe no reputable company would want to lead with this information but for my particular circumstances I would certainly be tempted to shop around for a test centre that was using a lower Ct value. I wouldn't go as far as using some company that was flagrantly flouting any lack of regulation by using a Ct value so low as to be effectively faking the testing by almost guaranteeing a negative result but if there was some reasonable range of Ct values considered by experts to be an acceptable range I would certainly want to be fishing at the lower "sensitive enough" end of the Ct value range rather than the higher "getting close to being overly sensitive" end of the range.

- Julian

Yours is and issue we frequently get asked about and it is a bit of a lottery in cases like yours. The most difficult are those who had covid perhaps two or three weeks before they wish to travel and it is highly likely that a PCR test may prove positive.

Our first advice is to take an antigen test if that is acceptable to the receiving country. Then we have a choice of four different tests we can use and with experience we have found some to be more sensitive than others.

John