This you may not legally refuse and one will be taken against your will if you do refuse. Fortunately, most police stations do not keep medical people on staff to do these tests so you will probably not get one unless you were arrested after a car accident and were taken to a hospital. This would not have been good for you if you tried your case later. In many instances it is better to admit having a couple of drinks. Make sure you stick to your number though.
Many people say a couple and before questioning is ended they are up to a six pack.
If your goal is to go home without arrest you better stick to your guns. You may have wondered why the cop shined his light in your face when he first got to the window. Officers are taught to do this to temporarily daze you so that they can briefly scan for weapons and contraband and get a whiff of your breath. They are looking for beer cans, open containers, weapons and illegal drugs. Anything in plain view may be catalyst for your eventual arrest.