Information on IHC and FISH testing is available on the home page of this website:
http://www.her2support.org/her2fish.pdf
IHC tests for the amount of Her2 protein there is in the cell, while FISH tests for the number of gene copies of Her2. There is high concordance between IHC +++ and FISH. FISH requires a special kind of microscope and is more "work for the pathologist," as my onc put it. The normal protocol is to use IHC testing, and if it is + it is negative, +++ positive, ++ indeterminate. The indeterminates are rechecked tested using FISH, which is considered definitive at that point.
Hopeful