There are a few tests that are used to diagnose GIST versus other cancers. There is mutation analysis to further determine which part of your DNA is causing the GIST. There are also more tests to diagnose liver tumors that may be metastasis.
from Dr. Anette Duensing:
The terms CD34, SMA, AE1/AE3, are pathological stains to help diagnose a GIST. CD34 is positive for most GIST, SMA is something that is positive in smooth muscle tumors (leiomyosarcomas) and is hence negative in GIST, and AE1/AE3 is positive in many carcinomas (and is hence also negative in GIST).
Ki-67 is a so-called "proliferation index" to estimate how quickly a tumor is growing. The higher the percentage, the higher the number of positive and hence dividing cells. This is somewhat similar to the mitotic count, but not exactly the same.
Anette (University of Pittsburgh Cancer Institute)