Reactive Thrombocytosis (Secondary Thrombocythemia) By Jane Liesveld, MD, James P Wilmot Cancer Institute, University of Rochester Medical Center Reviewed / Revised Jul 2022 | Modified Sep 2022 View Patient Education Reactive thrombocytosis is an elevated platelet count (> 450, 000 / mcL [> 450 × 10 9 / L]) that develops secondary to another disorder.