a friend of mine, he's a winner of the dv lottery of 2010, i asked him about this, and he said: between the date of interview and the medical examination, there is 15 days, so you have time to do the examination, once you received your interview letter go ahead and call the doctor assigned by them, it must be advised in the Interview letter which doctor to call, so you have to schedule an appointement with the doctor as soon as possible, the fees based on the experience of my friend is $150 it includes X-ray + blood test (HIV Sida).
good luck