Target CBC values in patients with ET on cytoreductive treatment

Dr Chris Hillis is an assistant professor in the Division of Oncology at McMaster University and hematologist at the Juravinski Hospital and Cancer Centre.

What are the target complete blood count (CBC) values in patients receiving cytoreductive treatment for essential thrombocythemia (ET)?

Chris Hillis, MD, MSc: For patients with ET the target values are different amongst the guidelines, so it makes it challenging to know in clinic what to actually target.

Most contemporary guidelines are recommending to target the platelet count into the normal range. However, most of the studies of cytoreductive therapy in ET were to target a platelet count <600 × 10⁹L. Unlike polycythemia vera (PV), where the risk of cardiovascular events is linearly correlated with the hematocrit, there is no direct correlation between the platelet count and the risk of cardiovascular events.

In clinical practice, while we do try and get into the normal range, we often will accept platelet counts <600 × 10⁹L. Of course, if the hematocrit starts to elevate, this patient may have had masked PV in the first place and you would want to maintain the hematocrit similar to what your target would be in PV, that is, <45%.

An even more controversial value would be what to do with the white count as it starts to elevate. We do know that the white count, as it elevates, is correlated with the risk of thrombosis and in some series also death, but we have no evidence that acting on the white count actually decreases this risk. But most folks who treat myeloproliferative neoplasms (MPNs) are starting to get uncomfortable when the white count rises to >20 × 10⁹L, and so we may begin to more aggressively cytoreduce somebody as the white count goes >20 × 10⁹L.