High concordance of SP3 rabbit monoclonal antibody with FISH to evaluate HER2 in breast carcinoma.

HER2 gene amplification or HER2 protein overexpression predicts a more aggressive clinical course in breast cancer, with a worse response to hormonal therapy, and determines eligibility for the use of the anti-HER2 antibody trastuzumab. For these reasons, the diagnostic assays that determine HER2 status in breast carcinoma have become increasingly important. Our goal was to evaluate the concordance, sensitivity, and specificity of a rabbit monoclonal antibody directed to the extracellular domain of the HER2 receptor (SP3) and compare it with fluorescence in situ hybridization and HercepTest in 179 invasive breast carcinomas. We found that SP3 was in agreement with fluorescence in situ hybridization results in 94.6% of cases. HercepTest and fluorescence in situ hybridization results were in agreement in 95.1% of the cases. Only 4.3% (4/93) of the cases that scored 0/1+ by SP3 were amplified by fluorescence in situ hybridization, and 8.3% (3/36) of cases that scored 3+ were not amplified by fluorescence in situ hybridization. Comparing SP3 with HercepTest, we observed that HercepTest demonstrated higher sensitivity (100.0% vs. 89.0%) but SP3 demonstrated higher specificity (97.0% vs. 89.0%). An important advantage of SP3 (in comparison with HercepTest) is its higher discrimination power (72.1% vs. 34.1%). For these reasons, this antibody could be helpful in the determination of HER2 status in a routine basis.