How do you approach a tumor that has been reported as HER2 amplified based on IHC 3+, but further testing with FISH is negative?   

Treat as Her2 positive. There is no need to do a FISH test if IHC is 3+. Positivity by either method has shown benefit.  

If IHC is on a core, then I would want to verify by FISH, and test resection specimen to avoid the pitfall of calling HER2+ disease when it is actually a false-positive, especially when we don't review path ourselves. Some clues are low grade, high ER and PR, low proliferation. If HER2+, these tend ...

I would start by repeating the test, and often at another lab. Just to be sure that the results are robust. We often see discrepancies from lab to lab. If confirmed, i would treat as Her-2 Positive if the IHC was truly 3+. Many of the patients that were enrolled on the trials that led to FDA approva...

ASCO/CAP just released their updated guidelines http://ascopubs.org/doi/full/10.1200/JCO.2018.77.8738