Tests not covered/prices not disclosed

I struggle with polycystic ovaries (PCOS)-insulin resistance goes hand in hand with this. Upon my 6 month follow up with my GP, she ordered various blood tests including Insulin levels to monitor if my medication is in effective doses. I immediately phoned Discovery to check if these tests were covered as I had moved to a different plan. I was advised that these test would just be deducted from my MSA (the conversation is recorded). I went for the tests only to find out that my Insulin test isn't covered and 50% of my MSA was used. This is not a large outstanding amount but I am surprised that I took the time to enquire about it and now there is a unpaid amount. I requested that they revise the claim and it was turned down again. The operator handling my call even expressed her disbelieve that they declined my claim. How is this not covered? Will I then have to advise my GP to monitor my health in another way? How is my insulin-lowering medication then covered? Is my disease not recognised by Discovery? I am very disappointed by this as this is a chronic condition needing chronic medical investigation-yearly blood test taking 50% of my MSA.