The article by Dr. Gilbert provides a comprehensive appraisal of the clinical aspects of essential thrombocythemia and polycythemia vera. The author reviews some of the previous work in the field that led to the current treatment recommendations, discusses newer treatment agents with platelet-lowering properties, and gives a balanced view of the attributes of both old and new treatment agents.
However, the diagnosis and management of polycythemia vera and essential thrombocythemia remain controversial. A lack of specific genetic markers has led to the development of complicated diagnostic criteria that are seldom used in routine clinical practice. I believe in a stepwise approach to reaching a working diagnosis rather than the application of a set of criteria that are not universally agreed upon. For example, one may suspect polycythemia vera if:
1. The hematocrit is above the upper limit of the reference interval (adjusted for sex and race);
2. There is a documented and persistent increase in hematocrit from a previous baseline; or
3. An upper normal hematocrit value is associated with a polycythemia vera-related feature such as aquagenic pruritus, large-vessel thrombosis, splenomegaly, erythromelalgia, microcytosis, or persistent leukocytosis or thrombocytosis.
Confirming a Diagnosis
In the absence of any of the above features, one can simply recheck the hematocrit in 3 to 6 months. However, if any of the aforementioned features are present, one can begin with a serum erythropoietin(Drug information on erythropoietin) determination and continue the investigation for polycythemia vera with a bone marrow examination. Otherwise, a repeat hematocrit and serum erythropoietin determination in 3 to 6 months is reasonable. A bone marrow histologic examination that is complemented by recently described immunohistochemical markers is valuable in supporting a working diagnosis of polycythemia vera. With this strategy, measurement of the red cell mass to confirm a diagnosis of polycythemia vera is largely unnecessary.
A similar stepwise approach may be applied to the diagnosis of essential thrombocythemia. One should first exclude the possibility of reactive thrombocytosis by reviewing the patient’s history, previous platelet counts, the peripheral blood smear, and levels of serum ferritin and C-reactive protein. In the presence of nonreactive and persistent thrombocytosis, other chronic myeloid disorders must be excluded before a diagnosis of essential thrombocythemia can be established. The possibility of polycythemia vera should be pursued if the hematocrit is near or above the normal reference interval. Otherwise, a bone marrow examination is indicated to rule out the possibilities of chronic myeloid leukemia, the myelodysplastic syndrome, and either cellular or fibrotic phase agnogenic myeloid metaplasia. In other words, essential thrombocythemia remains a diagnosis of exclusion.
In general, drug therapy is discouraged in low-risk patients (age < 60 years and no history of thrombosis and a platelet count of < 1.5 million/mL and the absence of cardiovascular risk factors) with either polycythemia vera or essential thrombocythemia because of the low incidence of associated thrombohemorrhagic complications.[5,6] However, there is good evidence to support the use of hydroxyurea therapy in high-risk patients (age ³ 60 years or a history of thrombosis) with either polycythemia vera or essential thrombocythemia. What is not clearly known is whether:
1. Indeterminate-risk patients (ie, patients who are neither high- nor low-risk) require cytoreductive therapy;
2. Other platelet-lowering agents may be substituted for hydroxyurea;
3. Hydroxyurea is leukemogenic in these disorders; and
4. Thrombocythemia is a risk factor for thrombosis in polycythemia vera.
Similarly, the target platelet count during the treatment of high-risk patients has yet to be defined. Hopefully, ongoing and future trials will provide definitive answers to some of these questions.
At present, I consider interferon-alpha (Intron A, Roferon-A) for polycythemia vera and anagrelide(Drug information on anagrelide) (Agrylin) for essential thrombocythemia as alternative, but not superior, therapies for young patients (age < 60 years).[9,10] These new agents can also be used with reasonable safety in older patients who are intolerant of hydroxyurea. In women of childbearing age, no specific treatment is recommended for low-risk disease, and interferon-alpha is the preferred agent for high-risk disease.
Finally, our group recently reviewed the long-term experience with anagrelide in patients with essential thrombocythemia and found that thrombohemorrhagic complications continue to occur in patients with platelet counts above 400,000/µL. Accordingly, we recommend a target platelet count of 400,000/µL in patients who require treatment.