Not a doc yet, but a veteran of 3 pregnancies and 2 years of med school...
It depends...Are you on oral contraceptives? Sometimes it takes longer than others for Aunt Flo to come by, especially if you have recently switched. Are you normally like clockwork, or do you sometimes go a day or two early or late? Is there a specific reason to worry? Have you had any symptoms (breast tenderness, nausea, bloating, utter exhaustion) more than normal for this time of the month?
Have you been under stress? Sometimes that throws even very normal cycles off, as can bad eating habits or over-exercising. There are lots of reasons it can be late, but if you are truly worried and want to know, the pregnancy tests work this early (I came up positive once BEFORE aunt flo was due!). Just remember that even if it is positive, this early on as many as 30 percent of pregnancies don't make it.
Hope things work out for you...