the whole concept of a "diagnostic" tests is bull**** in my opinion. Wow... lets take someone who hasn't prepared for one of the most difficult standardized exams and see how they do! Odds are you will not do well. But who care? No one. What does it mean? Not a damn thing. And as someone else mentioned, you've wasted valuable testing material.
98% of people are not going to do well until they are atleast ~70% of the way through their material review. Why waste exams when its highly probable you'll encountered problems/concepts you have not had a chance to review yet? You will just lose points that you WILL be able to get on test day... so your just harming the accuracy of your "practice" test results.
Get through all of your material quickly, but carefully. Then start doing practice exams while simultaneously doing a very thorough review of everything. That is the best approach in my opinion.