White blood cell count levels during 4 years of follow-up in 1638 patients with polycythemia vera. (A) The 10th, 25th, 50th, 75th, and 90th percentile of white blood cell count at the various time points. (B) The mean value of white blood cell count at the entry visit along with its 95% CI in patients belonging to the 10th and 90th percentile (time 0). Mean (95% CI) values of white blood cell count of these patients during follow-up (time points at 12 to 48 months) are maintained at levels similar to those observed at the entry visit.