Subnormal serum erythropoietin (EPO) level is widely accepted as a minor diagnostic criterion for polycythemia vera (PV) and has retained its value in the revised 2016 World Health Organization (WHO) classification scheme of myeloid neoplasms (Blood. 2016;127:2391). While over 80% of PV patients exhibit subnormal EPO values (Haematologica. 2004;89:1194), very little is known about the incidence, phenotypic connotation, or prognostic significance of serum EPO levels in essential thrombocythemia (ET).
Study patients were recruited from the Mayo Clinic, Rochester, MN, USA. Diagnosis of ET was in strict accordance with the 2016 WHO criteria (Blood. 2016;127:2391) and only patients with EPO measurements at diagnosis or during their chronic phase were considered under the following provisions: i) no evidence of myelofibrotic or leukemic transformation; ii) no treatment with cytoreductive agents at the time of sample collection (and for those with EPO sampling at diagnosis, no prior exposure to cytoreductive therapy); iii) no hemoglobin levels below sex-adjusted values for anemia; and iv) systematic exclusion of all cases of masked PV. Serum erythropoietin levels were measured using Mayo-verified standard immune-enzymatic assay consistent with CLIA requirements with reference range between 4-21 mIU/mL. Standard statistical methods and time-to-event curves prepared using the Kaplan-Meier method were used to compare EPO-stratified groups. The JMP® Pro 13.0.0 software package was used for all analyses (SAS Institute, Cary, NC, USA).
Among 177 informative ET patients (median age 51 years, range 18-88; 71% females), median serum EPO concentrations were 5.2 mIU/mL (range <1-25) overall and 2.6 mIU/mL (range <1-3.9) and 7.2 mIU/mL (range 4-25) in those with subnormal versus normal/above normal EPO levels, respectively (p<0.0001). The majority of EPO measurements were at the time or within one year of diagnosis (67%), with the remainder obtained at referral or during the chronic clinical course. Risk stratification disclosed low, intermediate, and high risk status in 45%, 33% and 23% of patients, respectively, with no appreciable differences between EPO-stratified subsets.
Subnormal serum EPO levels clustered clinically with JAK2 mutations (95% vs 60%; p<0.0001), higher hemoglobin (median 14.7 vs 13.8 g/dl, range: 12.1-17.8 vs 12-17.1; p<0.0001) and leukocyte counts (median 9.7 vs 8.2 x 109/l, range: 4-28.1 vs 3.9-22; p=0.0001), and arterial thrombosis after diagnosis (13% vs 5%; p=0.04). In addition, all three documented conversions to PV (p=0.01) and none of the six fibrotic progressions (p=0.02) occurred in patients with subnormal serum EPO levels (Table 1).
While serum EPO level had no bearing on overall survival data (p=0.41) (Figure 1A), subnormal circulating EPO concentrations were significantly associated with superior myelofibrosis-free survival (p=0.02) (Figure 1B) but conversely, inferior polycythemia-free survival (p=0.03) (Figure 1C).
We conclude that subnormal serum EPO levels are relatively frequent in ET, cluster with disease features that are reminiscent of polycythemia vera, and might be associated with a lower risk of fibrotic progression.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.