ELISAs, IHC and Fish are different.
http://en.wikipedia.org/wiki/ELISA
ELISAs are often scaled up, routine, in some respects. They go in a big plate with wells the size of paper punches. The antibody is stuck to the plate and your serum goes on with a probe, that if it binds to the plate, the well glows with a fluorescent probe, and can be run in huge numbers through an automatic reader.
IHC is Immunohistochemical, antibody stuck to a radioactive or color probe, more a test tube kind of reaction. Done with smushed tumor cells. The number is a concentration compared to normal.
Fish, is Fluorescent in situ Hybridization. They make a smear of your tumor cells. Smoosh the cells on a slide (or preserved in a thin sliced block) so the chromosomes are spread out. Then add a probe that makes the DNA glow just where the Her2 gene is. That way they can look at each cell, which proportion of the cells, how many copies of chromosome 17 in each cell. The >3 designation so many of us have means 3 or more copies of the her2 gene per cell.
Sorry, biologists answer.
Carla