PRV is suspected when an elevated hematocrit is found. Confirmation of an elevated red blood cell mass has been advocated as necessary but is becoming increasingly difficult to obtain in most laboratories, and there are concerns regarding its sensitivity. For females and males, a hematocrit above 50% and above 56%, respectively, is associated with an elevated red-cell mass, and this measurement can sometimes be used as a proxy for the red-cell mass. Secondary causes of polycythemia must always be excluded in order to diagnose PRV. These include any conditions associated with chronic hypoxia. Infrequently, an erythropoietin-secreting tumor may be found. The measurement of endogenous erythro-poietin may be helpful.
The diagnosis of ET is one of exclusion. On discovery of a consistently elevated platelet count of greater than 600,000, it is important to exclude iron deficiency anemia, infections, inflammatory disorders, and other neoplasms as the most common causes of secondary ET. The presence of the Jak-2 kinase mutation allows the diagnosis to be made in this and the other MPDs, apart from CML. However, a negative result does not exclude the diagnosis.
Chronic idiopathic MF is diagnosed by bone marrow examination. In the correct clinical situation of splenomegaly, cytopenias, nucleated red blood cells, teardrop-shaped red blood cells, and immature circulating WBCs, a diagnosis of marrow fibrosis is readily made. Cancer metastatic to the bone marrow can give a similar picture and needs to be excluded.
CML is diagnosed by the finding of the Philadelphia chromosome by conventional cytogenetics, preferably performed on a bone marrow sample or fluorescent in situ hybridization testing. PCR testing for the bcr-abl RNA transcript should also be obtained.
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