Study of interlaboratory reliability and reproducibility of estrogen and progesterone receptor assays in Europe - Documentation of poor reliability and identification of insufficient microwave antigen retrieval time as a major contributory element of unreliable assays

Immunohistochemical assays for estrogen receptors (ERs) and progesterone re ceptors (PRs) have not been surveyed for technical validity. In the present study, the reliability of the immunohistochemical assay for ER and PR was evaluated using data from 105 laboratories participating in external qualit y assessment (EQA) during a 2-year period. Technical variables associated w ith reliable immunostaining were analyzed. The efficiency of the antigen re trieval step was identified as the single most important contributory facto r influencing the overall reproducibility of the assays. Reliable assays we re found in 24 (36%) of 66 laboratories participating in continual EQA, inc luding the majority of centers known to have clinically validated results. Inadequate assay sensitivity with subsequent weak staining, was the main ca use of poor and variable results by laboratories using microwave antigen re trieval; too short a heating time was identified as the principal contribut ory factor: Extension of the heating time resulted in significant improveme nt regardless of all other variables in the immunohistochemical protocol. C ontinual participation in EQA is an effective means for identifying and ame liorating variables that influence the reliability of immunohistochemical a ssays for predictive markers, thereby assisting in technical validation and standardization.