[Interlaboratory trial 2000 " Immunohistochemistry" of the German Society for Pathology and the Professional Association of German Pathologists].

Early in 2000 an interlaboratory trial on immunohistochemistry was held in Germany in which 172 pathologists took part. Each pathologist received one H&E stained and five unstained slides of five different tumors to reach a diagnosis based on immunohistochemical stains. Additionally, the diagnosis-independent staining quality was assessed by using a multi-tissue block. Altogether, 828 diagnoses were made, among which 57% (468) were correct. The individual steps of immunohistochemistry (tentative morphological diagnosis, choice of primary antibodies, technical staining quality, conclusions from the diagnosis and rendering a final diagnosis) were assessed independently. Although each of these steps was correlated to the correct final diagnosis, in the multivariate analysis only the tentative diagnosis, choice of primary antibodies and the conclusions drawn from individual stains were independent factors to reach the correct final diagnosis. In the diagnostic part of the interlaboratory trial, the technical quality of the immunostaining was not an independent variable to reach a correct diagnosis. In contrast, the results of the multi-tissue block proved that the immunohistochemical staining quality has to be standardized to reach reproducible results in defining the estrogen receptor expression as a basis for therapeutic decisions.