The earliest for a pregnancy test to be positive?

See below: Conception, as defined by the generation of a fertilized ovum, occurs within day or so of the sex. However, it takes about two weeks for the embryo to make enough HCG to produce a positive pregnancy test. Blood test may be positive a day before a missed period. Home pregnancy tests should be done 4-5 days after the missed period. A negative test done too early is not useful.