We are flying from SFO to LHR and on to Rome on 9/12.

I have scheduled a PCR test on the 9th, am fully vaccinated.

As I understand it, UK needs the PCR test within 3 days of departure.
The fact that I am flying into LHR to make a connection- does that mean I am now entering Italy from the UK?
As I understand, UK visitors need a CPR test within 48 hours.

If anybody can help me with this would be awesome.
Thanks

I believe that, given you are coming into Italy from the UK (even though that is simply a pass-through), you will need to follow rules required as if you were coming from the UK.

From Italy's Local: "Arrivals from the UK will need to have taken either a molecular PCR test or a rapid antigenic test in the 48 hours before arriving in Italy, according to the ordinance."

Heathrow does have testing facilities if your US test does not meet this rule, but it sounds as if you need too book it ahead of time: https://www.heathrow.com/at-the-airp.../covid-19-test

Have you tried to check with your airline? If they board you in London, it is unlikely you will have any problems in Italy. From our experience last week, our airline was responsible for making sure passengers met criteria. There was no Covid police in Rome. In fact, our experience at FCO last week, there was no human contact with anyone at the airport and no one to show any vaccination or test results to.

Try using IATA’s Timatic with Italy as your destination country, and add United Kingdom as a transit country. That’s what the airline check-in staff use to check passengers’ documentation, and it’s kept up to date.
https://www.iata.org/en/publications/timatic/

Another good resource is the Reopen EU website/App. You just put in your departure and arrival countries and it gives you the most up to date information.

https://reopen.europa.eu/en

also, be sure you distinguish between # days before departure vs # days before arrival....

if your day of arrival in Europe is 9/12 then 9/9 would be 3 days. If you depart SFO on 9/12 then you arrive in Europe on 9/13...