From my own experiences, the doctors wouldnt prescribe meds until I did the blood test and had to return for the results. When I asked why he said people go and take the antibiotics straight away and then do the blood tests later on or the next day and this can give a false negative so that's why some doctors do it (I think this is how some WL cheat on the STD test that the shop makes them do, by taking a general antibiotic)
The way I stopped this from happening is by asking the doctor to give the meds but also tell him you understand exactly what's going to happen and why so you gotta promise to take the meds they gave you after you take the blood test.