I'm having a fasting blood test because they found gluclose in my urine but i'm also on trimethoprim for the uti. should i still go ahead with the test tomorrow morning?

Probably should wait: There are some reports of trimethoprim lowering blood sugar in patients already on diabetic medication. This affect is not as clear in normal patients. It is more important to have your UTI treated so I would suggest postponing your fasting blood glucose test until your trimethoprim is completed. The chance of affecting your glucose is low but you want your result to be accurate.