BACKGROUND: Laboratory testing of HER2/neu in breast carcinoma has become vital 
to patient care following the approval of trastuzumab as the first therapy to 
target the HER2/neu oncoprotein. Initial clinical trials used 
immunohistochemistry (IHC) to test for HER2/neu overexpression in order to 
select patients for therapy. Fluorescence in situ hybridization (FISH), which 
tests for gene amplification, is more specific and sensitive than IHC when 
either assay is compared with HER2/neu overexpression as determined by Northern 
or Western blot analysis. Many weak overexpressors on IHC testing are not gene 
amplified on FISH analysis. Such weak overexpressors may be considered 
false-positives and raise the question of how best to test for HER2/neu.
METHODS: The literature was surveyed regarding testing for HER2/neu 
overexpression in breast carcinomas and alternative testing strategies.
RESULTS: False-positive results are a significant problem when IHC is 
exclusively used to test for HER2/neu overexpression. The false-positives are 
overwhelmingly confined to the group of 2+ positives and do not respond to 
targeted therapy. In contrast, concordance between IHC and FISH is high when 
immunostaining is interpreted as either negative or strongly positive (3+). 
Whereas some recent studies have suggested that FISH may better predict response 
to anti-HER2/neu therapy than IHC, others have indicated that IHC is as 
effective a predictor as FISH. IHC is less technically demanding and costly than 
FISH.
CONCLUSIONS: IHC analysis of HER2/neu in breast carcinoma is a useful predictor 
of response to therapy with trastuzumab when strongly positive. Negative 
immunostaining is highly concordant with a lack of gene amplification by FISH. 
Most weakly positive overexpressors are false-positives on testing with FISH. 
Thus, screening of breast carcinomas with IHC and confirmation of weakly 
positive IHC results by FISH is an effective evolving strategy for testing 
HER2/neu as a predictor of response to targeted therapy.