Pediatric Polycythemia Vera Workup

Once polycythemia vera (PV) is suspected, the first step in evaluating a patient is determining whether the patient has primary, secondary, or apparent polycythemia.

A CBC count, ABG measurement, venous blood gas (VBG), and erythropoietin level can be used to differentiate patients. A CBC typically reveals increased leukocytes, platelets, and erythrocytes in primary polycythemia, whereas, in secondary and apparent polycythemia, only the erythrocytes are elevated. Primary familial congenital polycythemia (PFCP) is an exception; it only has elevated erythrocytes but not leukocytes or platelets. However, it can be distinguished from secondary polycythemia by its erythropoietin level. An erythropoietin (Epo) level is almost always low or low-normal in primary polycythemia; in secondary polycythemia, it is elevated or high-normal when hematocrit is high. Budd-Chiari syndrome in patients with the JAK2V617F mutation and elevated Epo levels has changed this absolute criteria in the diagnosis of polycythemia vera. An ABG reveals secondary appropriate polycythemia if it revealshypoxia. Finally,the VBG allows calculation of the P50 value; if this is low, it suggestsa highoxygen affinity hemoglobin or 2,3-bisphosphoglycerate (BPG) deficiency.

Ferritin levels may also help differentiate between primary and secondary polycythemias. Typically in primary polycythemia, the ferritin level is low due to constant overproduction of erythrocytes. In contrast, the ferritin level is usually normal in secondary polycythemia.

Red cell mass has been used to distinguish apparent polycythemia from secondary and primary polycythemia. However, the test is expensive and requires expertise. Also, the¹³¹ I-albumin used to measure plasma volume is not available in the United States and is difficult to handle because of its radioactivity. Consequently, the authors do not routinely use this test at the University of Utah School of Medicine because its diagnostic value is limited when the hematocrit level is clearly abnormal. The use of this test is frequently limited in clinical practice due to availability; however, some clinicians feel very strongly about it, especially in patients with borderline hemoglobin levels. It can occasionally identify a patient with an elevated red cell mass whose hematocrit is normal because of an increased plasma volume and can also identify patients whose hematocrit is only elevated due to a reduced plasma volume.

Secondary polycythemia must be differentiated into appropriate and inappropriate causes. As mentioned above, an elevated Epo level with a hypoxic ABG suggests secondary appropriate polycythemia, whereas an elevated Epo level without hypoxia suggests secondary inappropriate polycythemia. Determining the cause of appropriate polycythemia can proceed using the history, although specialized testing such as a p50 curve can be used to identify high-affinity hemoglobins due to structural hemoglobin defects or enzyme deficiencies. Hemoglobin electrophoresis is insufficient to identify hemoglobin structural defects because some hemoglobin mutants are missed. Secondary inappropriate polycythemia causes can be sorted out using judicious imaging and specialized endocrine testing.

Of the primary polycythemias, PFCP must be differentiated from polycythemia vera. These 2 diagnoses differ in clonality and in vitro responsiveness of peripheral blood erythroid progenitors to erythropoietin. Clonality testing relies on polymorphisms based on X chromosome inactivation and therefore can only be done in females. Polycythemia vera is clonal; PFCP is not. Endogenous erythroid colony responsiveness to Epo also differentiates polycythemia vera from PFCP. Polycythemia vera is characterized by growth independence from erythropoietin. In contrast, PFCP is not growth independent from erythropoietin, although it is hyperresponsive. Endogenous erythroid colony testing is not routinely available and can only be done in specialized laboratories. The authors frequently use it in our laboratory in difficult cases.

CT scan or ultrasonography of the abdomen can be used to assess the size of the spleen which is frequently enlarged in polycythemia vera.

Renal pathology, cerebellar hemangioblastomas, and pheochromocytomas that can cause secondary polycythemia may also be detected.

Bone marrow and aspirate in polycythemia vera tend to be hypercellular.

Some evidence of myelofibrosis may also be present.

In the plethoric phase, the blood smear shows normal erythrocytes, variable neutrophilia with myelocytes, metamyelocytes, and varying degrees of immaturity, basophilia, and increased platelets.

In the spent phase, the blood smear shows abundant teardrop cells, leukocytosis (or leukopenia), and thrombocytosis (or thrombocytopenia).

Because many practitioners do not have access to specialized clonality testing or erythroid colony assays, many of the criteria for diagnosis of polycythemia vera do not require them, although they are taken into account. No consensus has been reached on diagnostic criteria.

The World Health Organization (WHO) criteria for polycythemia vera diagnosis requires 2 components: reasonable elimination of apparent and secondary polycythemia and confirmation of polycythemia vera. However, the discovery of the JAK2V617F mutation have made these criteria insufficient. A proposed set of revised criteria have recently been published.

Diagnosis requires the presence of both major criteria and one minor criterion or the presence of the first major criterion together with 2 minor criteria.^{[13, 14]}

Major criteria

• Hemoglobin level of more than 18.5 g/dL in men, more than 16.5 g/dL in women, or other evidence of increased red cell volume (hemoglobin or hematocrit levels >99^th percentile of method-specific reference range for age, sex, altitude of residence; hemoglobin level >17 g/dL in men, >15 g/dL in women [if associated with a documented and sustained increase of at least 2 g/dL from the individual’s baseline value that cannot be attributed to correction of iron deficiency], or elevated red cell mass >25% above mean normal value).
• Presence of JAK2V617F or other functionally similar mutation such as JAK2 exon 12 mutation

Minor criteria

• Bone marrow biopsy showing hypercellularity for age, with trilineage growth (panmyelosis) with prominent erythroid, granulocytic, and megakaryocytic proliferation (not validated in prospective studies)
• Serum erythropoietin level below the reference range for normal
• Endogenous erythroid colony formation in vitro

Other groups have proposed and are preparing other potential diagnostic criteria. Criticism of the new WHO 2008 revised criteria revolves around the substantial interobserver variability in diagnosing polycythemia vera by bone marrow histology and the difficulty of community practitioners to test for endogenous erythroid colonies.