Influence of Platelet and White Blood Cell Counts until Major Thrombosis: Analysis from a Patient Registry in Essential Thrombocythemia

Retrospective studies could not proof a correlation of elevated platelet counts at diagnosis with thrombotic events. However, randomized and prospective clinical studies on platelet lowering therapies justify recommendations for the normalization of platelet counts as an appropriate endpoint for treatment in high-risk essential thrombocythemia (ET). Furthermore, there is increasing evidence that leukocytosis is an important risk factor for arterial thrombosis, especially in WHO classified ET. Leukocytosis is even more pronounced in prefibrotic primary myelofibrosis (PMF) compared to WHO classified ET. Patient cohorts that were diagnosed prior to the WHO classification likely consist of a mixture of ET and prefibrotic PMF. Some authors suppose interplay between white blood cells (WBC) and platelets in patients with myeloproliferative diseases.

The current study considers the Austrian cohort of a European registry, which was established to document the efficacy of the platelet-lowering therapeutic anagrelide in myeloproliferative neoplasms. Out of 845 patients, only those with a confirmed diagnosis of ET according to the PVSG classification until 2001 or according to the WHO classification thereafter were included in the analysis. Median follow up of the 620 included patients is 3.1 years (range 1.21-5.73 years), corresponding to 2428 patient years. 418 patients (67%) were females and the median age at study entry was 62 years (range 51-72 years). Thirty four patients (5.5%) experienced at least one major arterial or venous event until data analysis corresponding to 1.40 events per 100 patient years.

The influence of platelet and WBC counts on thrombotic risk was assessed by correlating the first major thrombotic event with the median of all blood counts recorded during the course of the disease until an event or until end of observation. The median platelet counts until the first event occurred and until end of observation (in patients without an event) were 595.8 G/l (range 495.0-743.0 G/l) and 500.0 G/l (range 410.5-619.5 G/l), respectively, and thus were substantially different (p= 0.008). The median WBC counts until the first event occurred and until end of observation (in patients without an event) were statistically not different but might indicate a trend with 9.60 G/l (range 8.70-10.95 G/l) and 8.42 G/l (range 7.05-10.70 G/l), respectively (p=0.084).

By using the calculated (Youden index) cut offs of 574.5 G/L for platelets and 8.48 G/L for WBC counts, respectively, the Cox regression analysis revealed a clear influence of platelets (p= 0.008) as well as WBC counts (p= 0.011) above the cut offs until the time to a major event. Interestingly, in this analysis, the influence of higher platelet counts on major thrombotic events was more pronounced than that of higher WBC counts.    

The frequency of major events per 100 patient years was highest when both platelet and WBC counts ranged above the calculated cut offs and was substantially different from the frequency when both, platelet and WBC counts ranged below the cut offs (3.05 vs. 1.4, p<0.001).    

In conclusion, our data add evidence to the impact of elevated platelet as well as elevated WBC counts on the time until a thrombotic event and on the frequency of thrombosis in ET. Moreover, we suspect a particular interaction between platelets and WBC which might not only be the result of simple addition of risk but might rather be based on biological interplay. We speculate that this interaction depends on particular cell counts of platelets and WBC which will need to be defined.