If you want to take eye exam to check up which you have every couple of years to check your eyesight and the health of the eye, you'd better take off your contact lenses. To get accurate prescription, you'd better take off your lenses and wait for two hours till you get eye checked. But of you just want to know how well your eyes in the contact lenses, you can wear contact lenses before eye exam.
Normally when you decide to receive an eye exam I guess you would like to know your real health condition of the eyes. So it is suggested to take off your contacts during the eye exam. Besides, the examer can detect whether you wear contacts or not, so it's better for you to get rid of them and it's also a way to show your honesty.
Honestly, you are suggested to take off you contacts before the eye exam. First, taking the goal of exam into account, we all know that it is to test the real condition of your vision. Thus if you wear contacts you will get a false result of that. I guess you don't want that. Generally the examiners are fond of those who obey the regulations and they may take those examinees more seriously. And then you will get a more correct result. So you are proposed to take off you contacts as it is benefit for your health.
Well, it looks like you are going to take an eye exam for something. As you know, such exam requires that your eyes are physically relaxed and ok. So it would be wise for you not to get your eyes strained, and you should try to take the exam early in thr morning, for then your eyes are quite suitable for checkups. Thus, you should get there without wearing your contact lenses.