Discrepancies in Clinical Laboratory Testing of Eligibility for Trastuzumab Therapy: Apparent Immunohistochemical False-positives Do Not Get the Message

Background: Several studies have reported what seem to be false-positive results using the Food and Drug Administration (FDA)-approved HercepTest (Dako Corp, Carpinteria, CA) to profile Her-2/neu amplification and overproduction in breast carcinoma. False-positive status has been based on comparisons with gene copy enumeration by fluorescence in situ hybridization (FISH) and with comparisons to immunohistochemistry (IMH) results using a monoclonal antibody. However, simple overexpression by tumor cells that have normal gene copy has not been evaluated by profiling mRNA expression, ie, such cases could simply represent true-positive, transcriptionally upregulated overexpression.

Materials And Methods: Four hundred infiltrating ductal carcinomas of breast were evaluated by IMH using monoclonal (CB11; Ventana Medical Systems, Inc, Tucson, AZ) and polyclonal (HercepTest; Dako) antibodies after antigen retrieval (AR). A polyclonal antibody sans AR (PCA/SAR) was also used. All IMH stains were evaluated and scored according to the guidelines for the FDA-approved HercepTest. A total of 145 of 400 carcinomas were subsequently evaluated by direct and digoxigenin-labeled (Dig) FISH, and 144 of 400 were evaluated by detection of mRNA overexpression via autoradiographic RNA:RNA in situ hybridization.

Results: Overall HercepTest/CB11 IMH discordance was 12%. Expression of mRNA was highly concordant with FISH and DigFISH amplification and with CB11 and PCA/SAR immunohistology. IMH false-positive cases (no Her-2/neu gene amplification) occurred with both HercepTest (23%) and CB11 (17%), and the majority of false-positive results (34 of 44) were scored as 2+. All 2+ false-positive cases were mRNA-negative. Combined results of HercepTest and CB11 showed that 79% (38 of 48) of 3+ cases were Her-2/neu gene amplified, but only 17% (seven of 41) of 2+ cases had increased gene copy.

Conclusion: Discordant HercepTest/FISH results, and to a lesser extent discordance with CB11 IMH, are most commonly false-positive results with a score of 2+. The 2+ score as defined in the guidelines for the FDA-approved HercepTest should not be used as a criterion for trastuzumab therapy unless confirmed by FISH. Determination of Her-2 gene copy number by FISH may be a more accurate and reliable method for selecting patients eligible for trastuzumab therapy.