Folic acid is also called Vitamin B9. It is water soluble and present in food. It does not cause any false negative or false positive effects on the drug test.
I would say there are some variables here you should mention. What are they testing for and how are they testing it. Vitamins are Amines (Vital Amines = vitamins) and wet tests are not very definitive. If your test is for opiates or meth those are amines as well. I would ask for a retest if you get a +. If the tests are done by GCMS they will likely be accurate but the tests in general should be Quantitative, not Qualitative.