Using a Glucometer on Shabbat

Rav Shlomo Zalman Auerbach zt"l permitted blood monitoring for diabetics, as is it considered a potential life threat.
However, in the case of a borderline diabetic, this depends on the severity of the diabetes. The activation of the machine involves usually a rabbinic prohibition, and is allowed even in non-life threatening situations with a shinui. [When not possible to use a shinui, it is permissible without.]
However, the lancing and taking blood involves a Torah prohibition and is permitted only when there is potential (even slight) concern of life.
Therefore, it is important for you to consult your diabetes doctor about the need for doing the test on Shabbat, and the necessary frequency. (It is possible that he may allow to skip the day, or suffice with less tests than usual.)
In your case, it also depends on how long you intend to take the Prednisone and the dosage you are taking.
There is a glucometer on the market that does not require piercing. It is called Freestyle Libre, marketed by Abbott. It is preferable to use such a system.

Rabbi Meir Orlian