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interpretation. Polydipsia and -uria and splenomegaly has been seen in animals assumed to have P. vera and rarely with other polycythemias.
Diagnostic Tests
As relative polycythemias are readily recognized, confirmation of a normal blood volume may not be required in clinical practice. In fact, patients with absolute polycythemia often have an expanded blood volume. Diagnostic tests for absolute polycythemia include
a complete blood cell count, absolute reticulocyte count, chemistry screen, urine analysis, blood gases and pulse oxymetry, chest and abdominal radiographs, cardiac examination and an ultrasound to evaluate
the kidneys and liver. Cyanosis may only be noted caudily. Dark blood may be exposed to air to determine if it is deoxyhemoglobin or methemoglobin. Serum erythropoietin values, determined by a species- validated assay, may be elevated in cases of secondary polycythemia, but a normal to low erythropoietin
level does not rule out secondary polycythemia. An increased absolute reticulocyte count in light of a polycythemia supports the exaggerated hematopoietic response and documents the presence of an absolute polycythemia. However, a bone marrow aspirate for cytologic examination adds no new information, since
it fails to differentiate between primary and secondary polycythemias: the myeloproliferative disease resulting in P. vera has no characteristic cytologic or pathologic features of malignancy
Treatment
The treatment of relative and absolute polycythemia
is clearly different. In emergency situations, patients with relative polycythemia will respond to fluid therapy, whereas patients with absolute polycythemia may need to be treated by phlebotomy. If possible, renal and other tumors/masses should be removed or treated and cardiopulmonary distress should be corrected. Animals with methemoglobin reductase deficiency may not need to be treated except during severe stress situations with methylene blue (1 mg/kg IV once). In severely polycythemic patients repeated phlebotomies at no more 20 ml/kg (10 ml in cats) per session with
or without simultaneous fluid replacement is the initial approach to lower the PCV to <60%. Phlebotomies can be repeated on a daily basis until the target PCV has been reached. In animals with absolute polycythemia, fluid administration beyond replacement may be associated with cardiopulmonary failure as these animals are completely volume expanded. In cases that require multiple phlebotomies in a short period, replacement of coagulation factors and albumin with plasma may need to be considered. The target hematocrit is higher in dogs than in cats, as well as in cases with cardiopulmonary disease, but generally is greater than 50%.
Longterm control of absolute polycythemia may
be achieved by periodic phlebotomies, radioactive phosphorus, and/or chemotherapy. Chemotherapy
with hydroxyurea at 10-25 mg/kg twice daily to once every other day is commonly used. Animals treated with chemotherapy need to be monitored, not only by PCV measurements as for phlebotomized patients, but also by complete blood cell counts to identify drug induced cytopenias. Hydroxurea has also caused nail sloughing. Furthermore, antithrombotic doses of aspirin (1 mg/kg per day) may be considered, but there is no proof that
it reduces the risk of thrombosis, and higher doses may lead to increased bleeding tendencies. Polycythemic animals may remain asymptomatic for weeks to years and, in some cases, can be successfully managed for years. Longterm follow up has not been reported in animals, although the author has observed persistent polycythemia for more than a decade in certain animals.
Author’s studies were supported in part by grants from the National Institutes of Health (OD010939) and the Winn Feline and other Foundations. The author is the director of the non-for-profit PennGen Laboratory which is offering genetic, hematological and blood typing and compatibility testing.
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