In this issueComparison of 5 Ki-67 antibodies regarding reproducibility and capacity to predict prognosis in breast cancer: does the antibody matter?â˜

- SP6, 30-9, poly, B56, MIB-1, and immunofluorescent-labeled MIB-1 antibodies were compared.
- Significant difference occurred between all Ki-67 LI assessments of the 5 antibodies.
- Highest concordance/agreement was observed between MIB-1 and poly, and 30-9 and poly.
- All investigated Ki-67 antibodies have prognostic potential except MIB-1-IF and B56.
- Only poly antibody was an independent predictor of prognosis at 20% threshold.

SummaryAlthough several antibodies are available for immunohistochemical detection of Ki-67, even the most commonly used MIB-1 has not been validated yet. Our aim was to compare 5 commercially available antibodies for detection of Ki-67 in terms of agreement and their ability in predicting prognosis of breast cancer. Tissue microarrays were constructed from 378 breast cancer patients' representative formalin-fixed, paraffin-embedded tumor blocks. Five antibodies were used to detect Ki-67 expression: MIB-1 using chromogenic detection and immunofluorescent-labeled MIB-1, SP-6, 30-9, poly, and B56. Semiquantitative assessment was performed by 2 pathologists independently on digitized slides. To compare the 5 antibodies, intraclass correlation and concordance correlation coefficient were used. All the antibodies but immunofluorescent-labeled MIB-1 (at 20% and 30% thresholds, P = .993 and P = .342, respectively) and B56 (at 30% threshold, P = .288) separated high- and low-risk patient groups. However, there were a significant difference (P values for all comparisons ≤.005) and a moderate concordance (intraclass correlation,  0.645) between their Ki-67 labeling index scores. The highest concordance was found between MIB-1 and poly (concordance correlation coefficient = 0.785) antibodies. None of the antibodies except Ki-67 labeling index as detected by poly (P = .031) at 20% threshold and lymph node status (P < .001) were significantly linked to disease-free survival in multivariate analysis. At 30% threshold, this was reduced to lymph node status (P < .001) alone. Our results showed that there are considerable differences between the different Ki-67 antibodies in their capacity to detect proliferating tumor cells and to separate low- and high-risk breast cancer patient groups.