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J Natl Cancer Inst. 2013 Dec 18;105(24):1897-906. doi: 10.1093/jnci/djt306. Epub 2013 Nov 7.

An international Ki67 reproducibility study.

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Affiliations of authors: Biometric Research Branch, Division of Cancer Treatment and Diagnosis, National Cancer Institute, Bethesda, MD (MCP, LMM); Pathology and Laboratory Medicine, University of British Columbia, Vancouver, British Columbia, Canada (SCYL, DG, EM, TON); Department of Laboratory Medicine and Pathology, University of Alberta, Alberta, Canada (JCH); Division of Pathology and Laboratory Medicine, European Institute of Oncology, Milan, Italy (MGM); Division of Pathology and Laboratory Medicine, European Institute of Oncology, and University of Milan, Milan, Italy (GV); Breakthrough Breast Cancer Research Centre, The Institute of Cancer Research, London, United Kingdom (LAZ); Department of Pathology, Centre Jean Perrin, Clermont-Ferrand, and Université d'Auvergne, France (FP-L); Transformative Pathology, Ontario Institute for Cancer Research, Toronto, Ontario, Canada (JMSB); PhenoPath Laboratories, Seattle, WA (AMG); Department of Pathology, MD Anderson Cancer Center, Houston, TX (WFS); Edinburgh Cancer Research Centre, Western General Hospital, Edinburgh, United Kingdom (TP); The EMMES Corporation, Rockville, MD (RAE); Breast Oncology Program, University of Michigan Comprehensive Cancer Center, Ann Arbor, MI (DFH); Academic Department of Biochemistry, Royal Marsden Hospital, London, United Kingdom (MD); on behalf of the International Ki67 in Breast Cancer Working Group of the Breast International Group and North American Breast Cancer Group.



In breast cancer, immunohistochemical assessment of proliferation using the marker Ki67 has potential use in both research and clinical management. However, lack of consistency across laboratories has limited Ki67's value. A working group was assembled to devise a strategy to harmonize Ki67 analysis and increase scoring concordance. Toward that goal, we conducted a Ki67 reproducibility study.


Eight laboratories received 100 breast cancer cases arranged into 1-mm core tissue microarrays-one set stained by the participating laboratory and one set stained by the central laboratory, both using antibody MIB-1. Each laboratory scored Ki67 as percentage of positively stained invasive tumor cells using its own method. Six laboratories repeated scoring of 50 locally stained cases on 3 different days. Sources of variation were analyzed using random effects models with log2-transformed measurements. Reproducibility was quantified by intraclass correlation coefficient (ICC), and the approximate two-sided 95% confidence intervals (CIs) for the true intraclass correlation coefficients in these experiments were provided.


Intralaboratory reproducibility was high (ICC = 0.94; 95% CI = 0.93 to 0.97). Interlaboratory reproducibility was only moderate (central staining: ICC = 0.71, 95% CI = 0.47 to 0.78; local staining: ICC = 0.59, 95% CI = 0.37 to 0.68). Geometric mean of Ki67 values for each laboratory across the 100 cases ranged 7.1% to 23.9% with central staining and 6.1% to 30.1% with local staining. Factors contributing to interlaboratory discordance included tumor region selection, counting method, and subjective assessment of staining positivity. Formal counting methods gave more consistent results than visual estimation.


Substantial variability in Ki67 scoring was observed among some of the world's most experienced laboratories. Ki67 values and cutoffs for clinical decision-making cannot be transferred between laboratories without standardizing scoring methodology because analytical validity is limited.

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