Yes, you can use other terms to turn them down, like credit score, income level, total people in the family. Discrimation means: if they are two applicants, ALL the other terms are the same, only one have health problem and another one don't, you turn down "health problem" one, then you will be sued.
Or can you say it is because they (with no health problem) start application early, that's why I decide to rent it to them because I ahve to follow the time line. Will that be OK?