National HER2 proficiency test results using standardized quantitative controls: characterization of laboratory failures.

Context: An important component in fostering test standardization for HER2 testing by immunohistochemistry is an appropriate positive control. We developed a new standardized, quantitative immunohistochemical HER2 control using a HER2 peptide covalently attached to glass microscope slides. The peptide controls can be formalin fixed or unfixed, providing the new capability of distinguishing errors associated with antigen retrieval from errors associated with the staining process itself.

Objective: To investigate the causes of variability in HER2 immunohistochemistry staining performance. By comparing laboratory performance with both formalin-fixed and unfixed analyte controls, we aimed to distinguish problems associated with antigen retrieval from reagent or staining protocol deficiencies.

Design: HER2 peptide analyte controls were printed on 2 slides that also contained unstained sections of invasive breast carcinomas and were mailed with the College of American Pathologists' 2006 HER2-B proficiency testing survey. Laboratory participants were asked to stain the 2 slides and return them for central review and quantification. This study is unique in combining central review with new quantitative HER2 controls.

Results: Of 109 participants who returned evaluable stained slides, staining was suboptimal in 20 (18.3%) as judged by quantification of the peptide analyte controls and review of tissue sections. Of those, 35% failed due to antigen retrieval errors, 20% failed due solely to antibody or staining protocol problems, and the remainder failed due to a combination of the two.

Conclusions: In practice, errors in HER2 testing are caused by variables associated with antigen retrieval and the reagents and staining protocol, as well as interpretive error. Analyte controls help distinguish these different causes.