Objective: The aim of this study was to evaluate the reliability of estrogen receptor (ER) and human epidermal growth factor receptor 2 (HER2) expression levels in Cellprep® (CP). Study Design: We evaluated the stability of immunocytochemistry (ICC) of ER and HER2 for primary or recurrent breast cancer samples rinsed in CP vials. Samples were prepared from CP vials stored for 1–30 or 160–240 days. ER and HER2 statuses were determined after 1–30 days (26 and 25 tests, respectively) or 160–240 days (15 and 18 tests, respectively) with the same protocols as immunohistochemistry (IHC), and were compared with the corresponding surgically resected specimens. Results: ER statuses according to CP samples showed perfect agreement (1–30 days: kappa, κ = 1; 160–240 days: κ = 1). HER2 statuses also showed good agreement (1–30 days: κ = 0.79; 160–240 days: κ = 0.64), although there were more equivocal HER2 cases in CP than in the surgically resected specimens. Conclusion: ER ICC in CP vials is reliable regardless of the preservation period. HER2 ICC in CP has more equivocal cases than HER2 IHC in surgically resected specimens. Both improvement of the immunostaining protocol and further validation study of in situ hybridization are indispensable for the practical application of ICC in CP.

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