4: Platelets

Platelets

Normal Morphology

Thrombocytopenia

Thrombocytosis

Platelet Clumps

Activated Platelets

Macroplatelets (Megaplatelets)

Megakaryocytes

Anaplasma platys (formerly Erhlichia platys)

Normal Morphology

Distinctive Features:

Platelets are cytoplasmic fragments that form from bone marrow megakaryocytes (platelet progenitors) and are the smallest formed element in peripheral blood. As platelets are cytoplasmic fragments, they have no nucleus and generally appear as light blue to light pink, round to oval structures, with few to many small reddish granules.

Diagnostic Significance:

One of the most important functions of platelets is the role they play in hemostasis, requiring both adequate platelet numbers and normal platelet function. Blood smear review is important in assessing the number of platelets as well as evaluation of platelet size, shape, and granularity. Canine and feline blood smears should have between 7 and 35 platelets on the average of ten 100× oil objective fields in the monolayer to be considered adequate. Feline platelets tend to become activated easily during collection, handling of the blood sample, or both; thus, feline platelets often clump, resulting in falsely decreased platelet counts and estimates.

Plate 4-1 Normal Platelets

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Thrombocytopenia

Distinctive Features:

Thrombocytopenia is the absence of platelet clumps and less than seven platelets per oil power field (100× objective) on an average for 10 or more fields.

Diagnostic Significance:

Be sure that platelet clumps are not present, as this will falsely decrease the platelet estimate, leading to an erroneous diagnosis of thrombocytopenia. If platelet clumps are not present, then each platelet seen on average in the 100× (oil objective) field equals approximately 15,000 to 20,000 platelets per microliter (µL).

Feline platelets are small and often similar in size to erythrocytes and may not be counted as platelets by automated hematology analyzers. Mild thrombocytopenia (100,000 to 200,000 platelets/µL) is not recognized on evaluation of a blood film. Only moderate and marked thrombocytopenias (platelet counts below 100,000/µL) are recognized. This is not a problem, since increased bleeding secondary to thrombocytopenia alone does not occur until platelet numbers are below 50,000/µL, and spontaneous bleeding secondary to thrombocytopenia alone does not occur until platelet numbers are below 25,000/µL.

Next Steps:

If platelet clumps have been ruled out and a true thrombocytopenia is evident, investigation for the underlying pathologic mechanisms for thrombocytopenia should be performed. Three main mechanisms of thrombocytopenia occur secondary to many different etiologies: (1) decreased platelet production (certain drugs and toxins, immune-mediated destruction of platelets, megakaryocytes, or both; neoplasia of the bone marrow); (2) decreased platelet survival (blood loss, disseminated intravascular coagulation [DIC], immune-mediated destruction); and (3) platelet sequestration (secondary to enlarged spleen). If a cause cannot be established, bone marrow cytology is often very helpful in determining if disease of the marrow is the cause of persistent, unexplained thrombocytopenia).

When evidence of clinical bleeding is present, and platelet numbers and coagulation times (prothrombin time [PT], partial thromboplastin time [PTT]) are normal, then investigation into platelet function defects should be considered with platelet function tests. Diseases resulting in altered platelet function may be both hereditary (Basset Hound thrombopathia, Chédiak-Higashi syndrome, cyclic hematopoiesis in Grey Collies, Glanzmann’s thrombasthenia of Great Pyrenees and Otterhounds), and acquired (drugs, toxins, infections, neoplasia, hyperglobulinemia).

Plate 4-2 Thrombocytopenia

Artifactual thrombocytopenia. A, Monolayer, devoid of platelets B, Feathered edge large platelet clumps

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Thrombocytosis

Distinctive Features:

Greater than 35 platelets per oil power field (100×) objective on an average of 10 or more fields.

Diagnostic Significance:

Physiologic thrombocytosis (increased platelet production) is the most common cause of thrombocytosis, occurring secondary to a reactive process (i.e., inflammation, iron deficiency anemia in dogs, rebound from thrombocytopenia, chronic blood loss anemia), with platelet counts typically fewer than 1,000,000 platelets/µL (<60 platelets per oil power field [100× objective] on an average of 10 or more fields). If the platelet count is persistently greater than 1,000,000 platelets/µL, then essential thrombocythemia and acute megakaryocytic leukemia should be considered, both of which are very rare hemic neoplasms of dogs and cats.

Plate 4-3 Thrombocytosis

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Tags: Atlas of Canine and Feline Peripheral Blood Smears

Jul 24, 2016 | Posted by admin in SMALL ANIMAL | Comments Off

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