A few years back there was a wonderful thread about "diagnostic exams" and the issues of putting too much credence into them. The crux of the argument was that for a test to be thorough enough to diagnose where your area of needs lie, it would need at least five questions, ranging from easy to hard, for each topic possible on the MCAT. After seven days of straight testing, you could grade all 10,000 questions, and from each set of five questions have an idea of where you stood on each of the 2000 subjects. This hopefully sounds absurd for a diagnostic, but it is a productive study plan if you review all of those questions.
An MCAT score depends on many things, from content knowledge to interpretation skills to application skills to test-taking logic. Confidence and anxiety also can have a major impact, but that's for another thread. Most people focus greatly on content knowledge when they start into their MCAT review. It offers comfort and provides a sense of moving forward, because after all, it's what you did for your college courses and it worked. People who are gifted at test taking achieve high MCAT scores by focusing on content, because they have already developed the other skills needed to do well. If you are one of these people, then you are golden. But the unfortunate thing is that most people are not like this, so when they emphasize content review, they miss out on developing the other vital aspects to do well on the test, and it often doesn't go well.
The point of my message in your case is that you need to realize that any score you get on an exam, diagnostic or not, reflects many things and you need to assess that before you commit so much attention to content review. Don't make the excess Anki mistake many people make. I agree with your idea that it would be a good idea to brush up on the topics you worry might be fading, chemistry and biology in your case, but do it by quizzing yourself as opposed to looking at flashcards or texts that are heavily-weighted on content over practice. Quizzing yourself results in better retention anyway, so you can get content review while developing test skills.
Good luck!