The first thing I would do is notify the Court or whoever it is that administers the test of your physical problems and circumstances and see how they want to handle it. Perhaps when such a thing happens, they will travel to you to administer the test. However, one thing I know for sure, when you simply don't show up, the Court immediately gets suspicioius that you're doing it purposely to hide something. So call them immediately or speak to a local lawyer to see what they suggest.
Jail is a possibilty in every violation of probation case. You should seek advice from an experienced local attorney about your facts, judge, probation officer, etc. Visit kliszlaw.com to discuss. Tim Klisz
Missed tests are often considered to be failed, or positive tests, for violation purposes. It would be better to have a lawyer explain the situation to the court.
Most judges, in my experience, would prefer that you come in and explain why you missed an ordered test than see a petition for a bench warrant. Likewise, probation officers or more inclined to work with you if legitimate circumstances prevent completing a required term of probation. If you've already failed to appear, make every effort to clear up those outstanding obligations. Again, a lawyer can help present your case, your efforts, and your obstacles quickly and in a language the courts can understand.