Effect of fixation time and microwave oven heating time on retrieval of the Ki-67 antigen from paraffin-embedded tissue.

Microwave oven heating has been employed for retrieval of antigens from formalin-fixed, paraffin-embedded tissue for immunohistochemical staining. Recently, a Ki-67 equivalent murine monoclonal antibody was generated which can detect tumor proliferative activity in routinely processed tissue with microwave oven heating. We assessed the effect of fixation time (4, 24, or 48 hr) and microwave oven heating time (7, 14, 21, 28, 35, or 49 min) on retrieval of the Ki-67 antigen from tonsil tissue. Ki-67 staining was quantitated by image analysis. Owing to the heterogeneity of Ki-67 staining within and between germinal centers, we employed a measurement technique that averages staining across the germinal centers of each section. Good Ki-67 immunostaining was observed for all microwave oven heating times in tissue fixed for 4 hr. In contrast, poor immunostaining was observed in tissue fixed for 48 hr unless a heating time of at least 14 min was used. Tonsil fixed for 24 or 48 hr showed a significant increase in percentage positive nuclear area after microwave times of 14 or 21 min. Prolonged heating time (up to 49 min) had no effect on the quality of Ki-67 staining in tissue fixed for 4 or 48 hr.