When do you become contagious?

[Happynewtier]

If someone was testing with the lateral flow tests (instant at home result) and tested negetive yesterday, but tested positive today, would they have been contagious yesterday?

Quite possibly yes.

According to test and trace you are contagious two days before your first symptom or positive test. At least that was what I was told this morning.

2 days before symptoms

Okay thank you! Wasn't sure if the contagious period just applied to when a person develops symptoms (hence gets a test) which would usually mean the positive result, after booking it, having it, and receiving the results, would probably make up the couple of days of being contagious and symptomatic, hence "being contagious two days before a positive result". The person in question is currently still asymptomatic, so just trying to work out the likelyhood of them being contagious yesterday when contact occured. Will be isolating regardless, but just trying to be prepared for the worst

As others have said, 2 days prior to onset of symptoms. If it is just a positive test and not symptoms today, it's possible that they were not shedding the virus yesterday. That said, they could have become positive or contagious immediately after a negative test so it's impossible to say for sure. The negative test only helps with that exact moment that the test was done and I'm not sure how sensitive the lateral flow ones are.

And obviously they could never develop symptoms and be positive anyway.

2 days before symptoms or a positive test. Lateral flow tests miss a lot (almost half) of people who are actually positive, so it could be that the yesterday your friend was negative or it could be that the test yesterday failed to detect covid even though it was present. That said not everyone who is in contact with a covid patient becomes ill themselves so you might be fine. You are definitely doing the right thing to isolate.

You can be contagious 48 hours before your first symptoms show.

And the best time to get a test is 5-7 days after exposure to the positive person. No sooner, as the virus may not show up.