<b>Comparison of Proliferation Markers Phosphohistone H-3 (PHH3) and Ki-67 in Determining True Mitotic Activity in Invasive Breast Carcinoma at PNS Shifa Hospital, Karachi</b>

Background: Accurate assessment of tumour proliferation is critical for grading and prognostication in invasive breast carcinoma. Conventional H&E mitotic counting is limited by interobserver variability, while Ki-67 lacks mitotic specificity. Phosphohistone H3 (PHH3) has emerged as a more reliable marker of true mitotic activity. This study compared PHH3, Ki-67, and H&E mitotic counts in breast carcinoma cases from PNS Shifa Hospital. Methods: This cross-sectional study included 80 cases of invasive breast carcinoma diagnosed between May and October 2024. H&E mitotic counts, Ki-67 labeling index, and PHH3 mitotic index were evaluated. Correlations, interobserver agreement, and impact on tumour grading were analyzed using SPSS v26. Results: PHH3 demonstrated the highest mean mitotic activity (14.6 ± 6.8/10 HPFs) compared with H&E (8.1 ± 4.6/10 HPFs) and showed strong correlation with H&E counts (r = 0.71, p < 0.001). Ki-67 showed moderate correlation with PHH3 (r = 0.62, p < 0.001). PHH3 upgraded 13.7% of Grade II tumours to Grade III, while Ki-67 upgraded 7.5%. Interobserver agreement was highest for PHH3 (κ = 0.84), supporting its reproducibility. Conclusion: PHH3 is a more sensitive, specific, and reproducible marker of true mitotic activity than Ki-67 or H&E. Its routine use may improve grading accuracy, particularly in borderline cases of invasive breast carcinoma. Further outcome-based studies are recommended.