Is a true false positive a reason for concern ? my tests are positive at home but negative through the blood?

Test for what?: If you mean pregnancy test, blood test is more reliable. Conception occurs within day or so of the sex. It takes about 3 weeks for the embryo to make enough HCG for a positive pregnancy test. Blood test may be done a day after a missed period. Home pregnancy tests should be done 4-5 days after the missed period. If the result is negative repeat the test in 4-5 days.