When does Covid show up in a test?

[JustSew]

Does anyone know whether you would test negative if you had contracted covid but not yet had symptoms?
A friend having urgent surgery has to have a test the day before. I would have thought she could still potentially have it, if she had been infected but not yet got symptoms?

[PeterWeg]

COVID test can be taken up to 5 days after symptoms appear. I suspect its not reliable outside that period.

[TabbyMumz]

30%of tests are not reliable. Testing is better done on days 1 to 3.