Inter‐laboratory and inter‐observer reproducibility of immunohistochemical assessment of the Ki‐67 labelling index in a large multi‐centre trial

Proliferative activity of tumour cells, as assessed by the Ki‐67 labelling index, has been suggested as a potential prognostic indicator in many neoplastic diseases. Meaningful application of the immunohistochemically determined tumour cell growth fraction in clinical decision‐making requires information about its inter‐laboratory reproducibility. To assess the reproducibility of Ki‐67 determined growth fraction, a multi‐centre immunohistochemical trial was performed with 172 participating laboratories, each testing 30 different tissue samples. Evaluating 5160 Ki‐67 labelling indices with a newly developed tissue microarray, good inter‐observer reproducibility but high inter‐laboratory variability was found. Reassessment of all stainings revealed considerable inter‐laboratory differences in the intensity and frequency of labelled nuclei, suggesting that antigen retrieval or staining techniques are predominantly responsible for the inter‐laboratory variability found in this trial. Consequently, cut‐off levels for Ki‐67, suggested to distinguish prognostic subgroups in tumours, appear to have limited reproducibility in a multi‐centre approach. It is concluded that there is a need to standardize the immunohistochemical determination of the Ki‐67 labelling index when it is used as a prognostic indicator in surgical pathology. Copyright © 2002 John Wiley & Sons, Ltd.