Causes of erythrocytosis and thrombocytosis are reviewed in Tables 56.1 and 56.2. Congenital erythrocytosis is extremely rare. Various mutations that disable the negative regulatory domain of the erythropoietin (EPO) receptor have been identified in autosomal dominantly inherited familial erythrocytosis. Congenital erythrocytosis may also result
Table 56.1 Classification of Erythrocytosis
EPO receptor hypersensitivity Hypoxia sensor hypersensitivity Acquired
Polycythemia vera Secondary
Appropriate (chronic tissue hypoxia) High altitude Pulmonary diseases Hypoventilation Cyanotic heart disease Smoking
Hemoglobinopathies: High oxygen affinity Congenital methemoglobinemia Inappropriate
Malignant tumors (renal, brain, liver, adrenal) Benign tumors (uterine fibroids, renal cysts) Androgen therapy Blood doping
Relative (normal red cell volume, decrease in plasma volume) Gaisbock syndrome
Differential Diagnosis of Erythrocytosis and Thrombocytosis 403
Table 56.2 Classification of Thrombocytosis
Myeloproliferative disorders Chronic myeloid leukemia Essential thrombocytosis Polycythemia vera
Myelofibrosis with myeloid metaplasia Secondary
Acute anemia (hemorrhage, hemolysis)
Malignancy from gene mutations that alter cellular sensitivity to hypoxia leading to inappropriate secretion of erythropoietin (EPO).
Polycythemia vera (PV) is the only acquired, clonal, primary cause of erythrocytosis. However, before diagnosing PV, it is essential to rule out other causes for erythrocytosis, which includes both secondary and apparent increases in erythrocyte volume. Apparent polycythemia may result from acute situations in which the plasma volume is greatly depleted (i.e., involving severe dehydration, diarrhea, vomiting, aggressive diuresis, severe burns) and the hematocrit becomes elevated. In addition, some otherwise healthy people will have hematocrits that are slightly above the upper limit of the reference interval, due to decreased plasma volume rather than increased red cell volume (apparent erythrocytosis). This condition is poorly understood, but is associated with obesity, hypertension, and smoking, and does not warrant an evaluation for PV in the absence of other diagnostic features of the disease as discussed below.
Secondary polycythemia represents a true increase in erythrocyte volume. Causes include chronic exposure to carbon monoxide as is seen in long-term smokers, chronic hypoxia due to cardiopulmonary diseases, obstructive sleep apnea or living at high altitudes, erythropoietin (EPO)-producing tumors, and congenital high-oxygen-affinity hemoglobins, to name only a few.
Congenital thrombocytosis is also extremely rare. Typically, acquired thrombocytosis is a reaction to an underlying process (Table 56.2), even when the platelet count exceeds 1000 x 103/mL. If secondary causes are not identified, a diagnosis of a myeloproliferative disorder requires further evaluation to differentiate primary thrombocytosis from chronic myeloid leukemia, primary myelofibrosis with myeloid metaplasia, and polycythemia vera since the prognoses and treatments are different.
In the case presented above, there were no apparent secondary explanations for either the thrombocytosis or erythrocytosis, and the preliminary diagnosis was polycythemia vera. First, poycythemia was confirmed by performing red cell and plasma volume determinations in the nuclear medicine department. Venous blood was sterilely collected, and known quantities of 51Cr, to label red cells, and 125I-labeled albumin were added. Following reinfusion of the labeled whole blood, a second sample was collected and isotope concentration measured. Based on the degree of isotope dilution, red cell volume and plasma volume were calculated. The patient's red blood cell volume was 38.9 mL/kg (reference interval 18.9-31.6 mL/kg), and plasma volume was 36.6mL/kg (reference interval 27.7-46.2 mL/kg), confirming an absolute erythrocyctosis. Findings from a bone marrow biopsy were typical for polycythemia vera: a hypercellular marrow, with expansion of myeloid, erythroid, and megakaryocytic lineages with abnormal clusters of megakaryocytes; no evidence of myelodysplasia; and no stainable iron. Finally, DNA testing revealed that the patient was positive for the JAK2-V617F mutation. This further supported the diagnosis of polycythemia vera in this patient.
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