Three-minute peripheral blood film evaluation: The erythron and thrombon - Veterinary Medicine
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Three-minute peripheral blood film evaluation: The erythron and thrombon

If the platelet numbers are decreased, can the mechanism be determined?

Thrombocytopenias occur through four basic mechanisms: sequestration, utilization (consumption), destruction, and decreased or ineffective production. While clues as to the underlying mechanism are found in the CBC, in most cases, bone marrow evaluation is needed for complete interpretation.

Sequestration thrombocytopenias are uncommon in veterinary medicine. They are usually the result of hypersplenism and, thus, are characterized by an enlarged spleen on physical or radiographic examination.

Utilization, or consumption, thrombocytopenias are caused by excessive activation of the coagulation cascade. These thrombocytopenias are associated with inflammatory disease and DIC. Utilization thrombocytopenias are usually moderate, with platelet numbers in the 75,000 to 150,000/l range, but values below 40,000/l accompanied by petechiae are possible.10 Bone marrow evaluation reveals normal to increased numbers of megakaryocytes.

Destruction thrombocytopenias are immune-mediated thrombocytopenias in which normal circulating platelets are destroyed by circulating antiplatelet antibodies. Destruction thrombocytopenias are often extreme, with peripheral platelet counts well below 50,000/l. Bone marrow examination is characterized by normal to increased numbers of megakaryocytes.

Decreased or ineffective production is associated with extremely low platelet counts; counts well below 50,000/l are common. Bone marrow evaluation will vary greatly in most of these cases, although they will share similar end results of deceased effective production by the bone marrow. With decreased production thrombocytopenias, no identifiable to extremely low numbers of megakaryocytes are present. With ineffective production, most bone marrow samples have normal to increased numbers of megakaryocytes, and the production of platelets is ineffective in many cases because of an immune-mediated destructive process directed at an early stage of platelet development or at the megakaryocyte population itself.

If platelet numbers are increased, is the thrombocytosis reactive or neoplastic?

Most thrombocytosis is reactive or neoplastic. Reactive thrombocytosis can occur secondary to exercise, hemorrhage, splenectomy, excitement, fractures, high circulating glucocorticoid concentrations, myelofibrosis, and iron deficiency anemia as well as 24 hours or more after blood loss.11 When the possible causes of reactive thrombocytosis have been ruled out, then the possibility of primary platelet leukemia must be considered. When extremely high platelet counts are seen (> 1,000,000/l), thrombocytosis due to neoplasia must be strongly considered.

Are enlarged platelets present?


17. A blood film from a dog with immune-mediated hemolytic anemia. Note the enlarged platelets (black arrows). Also note the polychromasia, spherocytosis, and Howell-Jolly body (red arrow) (modified Wright's stain; 100X).
The presence of enlarged platelets (Figure 17), which correlates with increased mean platelet volume (MPV), is supportive of an increased rate of thrombopoiesis in the bone marrow in response to a peripheral demand. This interpretation can be used with most species; however, in cats, enlarged platelets is an equivocal finding.

CONCLUSION

Determining if an anemia is regenerative or nonregenerative is critical when developing a list of differential diagnoses. In addition, automated platelet counts should be verified by examining blood films because platelet clumping is common, especially in cats, which results in artifactually decreased platelet counts.

Fred L. Metzger Jr., DVM, DABVP (canine and feline practice)
Metzger Animal Hospital
1044 Benner Pike
State College, PA 16801

Alan Rebar, DVM, PhD, DACVP
Department of Veterinary Pathobiology
School of Veterinary Medicine
Purdue University
West Lafayette, IN 47907

ACKNOWLEDGMENT

The authors gratefully acknowledge the technical assistance and images provided by Dennis DeNicola, DVM, PhD, DACVP.

REFERENCES

1. Rebar A.H. et al.: Laboratory methods in hematology. A Guide to Hematology in Dogs and Cats. Teton New Media, Jackson, Wyo., 2002; pp 3-36.

2. Duncan, J. et al.: Erythrocytes. Veterinary Laboratory Medicine, 3rd Ed. Iowa State University Press, Ames, 1994; pp 3-61.

3. Feldman, B.F. et al.: Reticulocyte response. Schalm's Veterinary Hematology, 5th Ed. Lippincott Williams & Wilkins, Baltimore, Md., 2000; pp 110-116.


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