Trend of circulating CD34(+) cells in patients with myelofibrosis: Association with spleen response during ruxolitinib treatment

We evaluated CD34(+) cells in a single‐centre series of 49 consecutive patients with myelofibrosis (MF) at baseline and during ruxolitinib therapy and examined any association with spleen response. The median (range) absolute number of circulating CD34(+) cells was 0.0835 (0.001–1.528) × 10(9)/L at diagnosis, and 0.123 (0.002–1.528) × 10(9)/L at ruxolitinib start. With the exception of a transient increase after 3 months of ruxolitinib therapy, a progressive reduction in CD34(+) cells count was documented, down to a minimum of 0.063 × 10(9)/L after 36 months. We then assessed the association between spleen diameter expressed as the distance from the left costal margin (outcome) and log(CD34(+)) cells count using random‐intercept and random slope multivariable regression models to take into account within subject correlation: after adjusting for time and ruxolitinib dosage, we estimated a 0.7 cm increase (95% confidence interval 0.2–1.2, p = 0.003) in spleen length for each unit increase in log(CD34(+)) cells count (× 10(9)/L). Although our study has some limitations, mainly related to its retrospective design, our approach may introduce a reproducible and simple tool that could facilitate the assessment of spleen response more objectively in patients with MF treated with ruxolitinib.