Originally Posted by
Often1
The pre-departure test for the US may be as far as 3 calendar days prior to departure. Thus, as early as Day 4. So long as the third "inbound" test meets the requirements for US entry, OP can use it for that. A quick look suggests that the UK post-arrival tests do meet the CDC Order requirements for the US (as well they should).
I know the pre-departure test going back to the US can be an anti-gen (and taken at LHR) how about the test required for entry into the UK--does that have to be a PCR? I would think that if I get a test 24 hours before entering the UK I will use those results to satisfy day 2 and since I'm leaving day 7 I won't need (obviously) day 8.