RESUMEN
Assessment of HER2 biomarker in invasive breast carcinoma patients allows a specific therapeutic approach. Clinical guidelines indicate immunohistochemistry (IHC) and in situ hybridization (ISH) to test HER2, however both have drawbacks which results in low reproducibility of results especially in equivocal cases. Our main objective is to quantify inter-observer IHC reproducibility and cross it with the ISH result. Our series includes 205 invasive breast carcinoma cases sent for ISH retest from 14 hospitals, 5 observers to assess the IHC and 1 observer for ISH of each case. We found that the observers only achieve an absolute agreement for IHC in 1 out of 3 cases. The inter-observer concordance for IHC is low (0.2 ≤ k ≤ 0.4) or moderate (0.41 ≤ k ≤ 0.6). In ISH positive cases the concordance for IHC is higher than in the ISH negative cases. In conclusion, the study shows low and moderate IHC inter-observer concordance, finding the more worrying values among the ISH negative cases which are the most part of this particular sample. Subjective interpretation of the techniques, among other factors, has negative impact in HER2 evaluation. To offset this limitation we have checked that reaching a consensus from different observers for HER2 IHC assessment improves the results.