The highly informative article from the Gruppo Italiano Studio Policitemia [1] has increased our knowledge of the course of polycythemia vera. Although we endorse most of the author's conclusions, we believe the study has several limitations inherent in its retrospective design.

Although the diagnostic criteria established by the group are useful in the research setting, they tend to be restrictive and may exclude as many as one third of patients who have polycythemia vera but incomplete presentations [2]. In interpreting the Italian group's findings, it seems plausible that some or most of the patients who previously had thromboses were actually presenting at that time with an incomplete form of polycythemia vera. Given the insidious course of the disease, such patients may differ in terms of neoplastic complications from patients who meet the above criteria at the first manifestation of the disease. Thus, they should be analyzed separately. Moreover, the approach to treating these milder cases may have been less aggressive in terms of myelosuppressive agents. Therefore, the outcome in patients with mild disease may have differed considerably with respect to neoplastic complications.

The authors conclude that survival duration in patients with polycythemia vera is longer than previously believed, a result similar to those from several other retrospective studies. Retrospective assessment of survival poses several difficulties. For example, sudden death (such as that from massive myocardial infarction) occurring before diagnosis is established would exclude a patient from the cohort. Therefore, such studies tend to overestimate survival. Another variable affecting survival is that patients who receive a diagnosis earlier in the course of their disease tend to have a longer arithmetic survival. For example, in another study [2] in which 16% of patients with polycythemia vera were identified incidentally while asymptomatic, the overall median survival was 17.4 years.

Our final criticism relates to the conclusion that myelosuppressive agents have an overall unfavorable effect. In their Discussion section, the authors contend that patients receiving myelosuppression therapy may have more severe disease, as supported by the higher frequency of thrombosis. Such potential selection bias precludes a separate assessment of the adverse effects of myelosuppressive agents.