HER-2 protein concentrations in breast cancer cells increase before immunohistochemical and fluorescence in situ hybridization analysis turn positive. 
The level of HER-2/neu in breast cancer cells is normally measured by immunohistochemistry (IHC) and/or fluorescence in situ hybridization (FISH).  It determines whether patients should be treated with trastuzumab (Herceptin).  In this study, HER-2 protein in breast cancer tissue was measured using a quantitative method.  Tissue samples of malignant and adjacent benign breast tissue were collected from 118 consecutive women admitted for surgical treatment of breast cancer.  The HER-2 protein concentration was determined by 2 HER-2 assays: ELISA and the Bayer ADVIA Centaur assay.  Paraffin-embedded tissue sections of the corresponding tumors were analyzed by IHC and FISH.  Increased HER-2 concentrations in cancer tissue were found compared to autologous reference tissue (p<0.0001, Wilcoxon test) and normal breast tissue (p<0.0001, Mann-Whitney test).  Good concordance rates were observed between the methods: 95.8% for IHC and FISH; 86.4% for IHC and ELISA; and 87.3% for FISH and ELISA.  The HER-2 positivity rate was determined to 26.3% by ELISA, 12.7% by IHC and 16.9% by FISH.  No correlation was found with tumor grade, axillary node status or serum HER-2 levels.  Detection of HER-2 overexpression by measuring HER-2 in tissue extracts by ELISA seems to be more sensitive than IHC and FISH.  This suggests that some patients deprived of Herceptin treatment may benefit from this treatment and may also explain the conversion phenomenon from HER-2-negative to HER-2-positive observed in relapse and metastatic disease.