AbstractIntroductionCellprep® (CP) is a novel liquid‐based cytology (LBC) system. This study aimed to assess the immunoreactivity of control samples stored long‐term under two storage conditions using CP.MethodsThe immunoreactivity in control samples was evaluated under two sample storage conditions: CP vial storage at room temperature (RT: 20–25°C) and CP slide storage in a refrigerator (2–6°C). Clinical samples as controls (total: positive/negative) were immunostained using thyroid transcription factor‐1 (TTF‐1) (20: 14/6), GATA binding protein 3 (GATA‐3) (13: 10/3), carcinoembryonic antigen (CEA) (23: 15/8), and the p16/Ki67 cocktail (20: 12/8) markers. The first immunocytochemistry (ICC) was performed within 1 month using CP vials stored at RT. Samples stored in CP vials and on CP slides were used for the second (within 3–6 months) and third (within 6–11 months) ICCs. Compared with the first ICC, the concordance of immunoreactivity for the second and third ICCs was evaluated using the weighted kappa coefficient.ResultsFor TTF‐1, CEA, and the p16/Ki67 cocktail markers, ICC controls had stable immunoreactivity for a minimum of 6 months when samples were stored in CP vials (kappa coefficients >0.8), whereas for GATA‐3, they were for 3 months. On CP slides, only for the p16/Ki67 cocktail, ICC controls had stable immunoreactivity for at least 3 months (kappa coefficient >0.8).ConclusionClinical samples as ICC controls revealed consistently more stable immunoreactivity in CP vials than on CP slides for TTF‐1, GATA‐3, CEA, and the p16/Ki67 cocktail markers.