Dr h. i am having symptoms that could potentially due to hiv or syphilis. full body rash after sexual encounter. would testing for hiv/syphilis now (37 days) rule out whether my symptoms are due to hiv / syphilis or do i need to wait the full 45 days?

For both HIV and syphilis, negative blood tests overrule any and all symptoms. It isn't possible to have HIV or syphilis symptoms with negative blood test. You need to wait til 45 days for conclusive testing to prove you aren't infected, but the negative results already prove your symptoms are not caused by either HIV or syphilis. See a doctor if they continue and/or you remain concerned.