Assessing Longitudinal Trends in Essential Thrombocythemia, Polycythemia Vera Blood Counts

In essential thrombocythemia (ET) and polycythemia vera (PV), routine measurements of hematological parameters are crucial to diagnosis, prognosis, and monitoring. A recent study published in the British Society for Hematology journal eJHaem aimed to assess how well hematological targets are met and maintained in practice for patients with ET and PV.

ET and PV have differing diagnostic criteria, with ET characterized by increased platelet counts and PV characterized by high hemoglobin, hematocrit, and/or red blood cell mass. The goal of both ET and PV treatment is generally to lower the risk of vascular complications and disease progression to myelofibrosis or acute myeloid leukemia (AML).

Ideal hematological targets have been debated, but hematocrit lower than 45% in PV and a platelet count at or below 400 × 109/L in ET are generally accepted as levels that reduce the risk of thrombotic events in both conditions and reduce the risk of hemorrhagic events in ET. In both ET and PV, a white blood cell (WBC) target lower than 10 x 109/L is the recommended threshold. These criteria, which are based on the European LeukemiaNet recommendations, were used in the study.

In all, 620 patients with ET and 429 patients with PV were identified. Data from 2008 to 2020 were collected from the Chelsea and Westminster Hospital NHS Foundation Trust and the Oxford University Hospitals NHS Foundation Trust in the UK. Researchers modeled longitudinal changes in patient blood counts, including those who met peripheral blood count remission.

Overall, approximately 85% of patients had blood count data from within 30 days prior ro or after the estimated index date. Patients with ET had median platelet levels of 524.0 × 109/L (interquartile range [IQR], 382.3-681.9) and median WBCs at 10.3 × 109/L (IQR, 7.8-12.8) at the time of diagnosis. Patients with PV had high median hematocrit, hemoglobin, and red blood cells (RBCs) at cohort entry ([47.5%; IQR, 43.4-51.6], [15.6 g/dl; IQR, 13.7-17.2], and [5.5 × 1012/L; IQR, 4.8-6.1], respectively). A total of 109 (17.6%) patients with ET and 53 (12.4%) patients with PV had histories of hemorrhage or thrombosis.

Further analysis of high-risk patients included 417 (67.3%) patients with ET and 288 (67.1%) with PV. Of those patients, 374 (90%) and 262 (91%) had at least one recorded blood count over 5 years and were part of the longitudinal analysis. Higher average blood count measurements were generally seen closer to index date and became more stable at 3 to 12 months post-index.

In patients with ET, platelet counts decreased the most over time. They typically started high at index (604.6 × 109/L [95% CI, 497.0-735.5] in an average 60-year-old female), would reduce to 337.7 × 109/L (95% CI, 278.0-410.1) by 1 year, and rise slowly to 436.6 × 109/L (95% CI, 330.6-576.7) 5 years. Consecutive counts could vary between measurements, the authors noted. Hematocrit and hemoglobin only decreased marginally, but RBC dropped from 4.6 × 1012/L (95% CI, 4.3-4.8) at index date to 3.9 × 1012/L (95% CI, 3.7-4.2) at year 1.

Platelet count declines in PV were less pronounced, with an average 60-year-old male patient dropping from 323.8 at index (95% CI, 290.7-360.6) to 268.8 at year 1 (95% CI, 242.1-298.5). Declines in hemoglobin and hematocrit were significant in the first month and continued to decrease until year 1.

Overall, 80.5% and 78.6% of patients with high-risk ET and PV, respectively, achieved complete peripheral blood count remission at least once over the observation period. High-risk patients often moved in and out of remission, with ET patients in remission for 46.4% of the time based on platelet counts and 76.3% based on WBC counts. PV patients were in remission for 55.4% of the time based on hematocrit levels, 71.5% based on platelets, and 69.0% based on WBC count.

Achieving remission was associated with a lower risk of clinical outcomes, but the small number of events that occurred in the cohort did not show statistically significant associations between peripheral blood count remission and cardiovascular events.

Study limitations included its observational nature and its focus on high-risk patients assumed to be treated with best clinical practices. Retrospective data from electronic records also makes it difficult to account for patients lost to follow-up.

Overall, the findings add real-world data to provide insight into hematological progression in ET and PV patients over 5 years.

“Although these findings highlight the challenge in achieving peripheral blood count remission in ET and PV in routine care, the extent to which meeting these thresholds translates to improved outcomes for patients is less clear,” the authors wrote.

Reference

Carpenter L, Rockenschaub P, Hatton G, et al. Longitudinal trends of peripheral blood counts in polycythaemia vera and essential thrombocythemia patients in the UK. EJHaem. Published online June 29, 2022. doi:10.1002/jha2.519