Late AF, BFN

So my AF is late (day 3), I have taken several tests and all negative. How long should I wait before I go and get blood test done? We are TTC #3. My last pregnancy was unpredictable with AF but it still came (sometimes lasting 3 days, sometimes 5 days). I had a negative test then but thought I had a urinary tract infection and we to doc. Did have UTI and was prego. Can a UTI effect tests?