Overcoming the diagnostic and therapeutic challenges of canine immune-mediated thrombocytopenia - Veterinary Medicine
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Overcoming the diagnostic and therapeutic challenges of canine immune-mediated thrombocytopenia
Determining that the immune system is the cause of a dog's decreased platelet count can be difficult. These clinicians walk you through the diagnostic and treatment process so you can help patients with this life-threatening bleeding disorder.


VETERINARY MEDICINE


Initial sample collection

At presentation, stabilize the patient, if necessary, and then initiate diagnostic testing. But before initiating therapy, collect and save EDTA, sodium citrated, and serum samples.6 Use these samples to perform a CBC with blood smear evaluation, a serum chemistry profile, a coagulation profile, heartworm antigen testing, and a crossmatch, if appropriate, as well as to measure antibody titers for infectious diseases. Collection by atraumatic venipuncture is necessary to avoid activating tissue factor within the needle. Process the samples within a few hours of collection to minimize platelet clumping. If severe thrombocytopenia is suspected, it is vital that the patient be handled as carefully as possible to avoid iatrogenic hemorrhage.

Evaluating the CBC and blood smear

Confirm thrombocytopenia with a CBC that includes an automatic platelet count and a blood smear examination.6 Reference ranges for platelet counts vary slightly among laboratories but are typically between 200,000 and 500,000/μl.1 Healthy greyhounds and possibly Shiba Inus have lower platelet numbers than other breeds.16 Spuriously low platelet counts can be due to clumping. A low count can also occur when macroplatelets are present and overlap with the size of erythrocytes. Most bench-top hematology analyzers use cell size, measured by impedance technology, to distinguish platelets from erythrocytes. So when they overlap, the analyzer has difficulty distinguishing the cell type, and platelets can be counted as erythrocytes. Consequently, low platelet counts should always be verified by examining a blood smear.2,6

A quick way to evaluate the number of platelets on a blood smear is to count the number of platelets per high-power field (HPF) (100X). Average the number of platelets in five to 10 fields, and then multiply the average number of platelets per HPF by 15,000/μl to calculate the total platelet count.6-8 More than 12 platelets per HPF represents a normal count.7,8 In addition, examine the feathered edge of the smear for platelet clumping. If platelet clumps are present, the platelet number assessment will be inaccurate.6,8 If clumping cannot be prevented, we recommend submitting the slide to a clinical pathologist for review and estimation of whether platelet numbers are adequate.

Also assess platelet size, as large, immature platelets (macrothrombocytes) indicate bone marrow response and have increased hemostatic function.1,6 On the other hand, small platelets (microthrombocytes) are suggestive of immune-mediated platelet destruction.2,5 However, platelets swell in EDTA, which may obscure this finding.5 Mean platelet volume (MPV) reflects the average size of platelets in circulation.4

The likelihood of hemorrhage increases when the platelet count is below 50,000/μl. However, spontaneous hemorrhage is uncommon until the platelet count is below 30,000/μl, unless a concurrent disorder of primary hemostasis, such as a platelet function defect or vasculitis, is present.1,6 Reticulated platelets, which are platelets recently released from the bone marrow, may be quantitated by using flow cytometry as an indication of thrombopoiesis, but this test is not routinely evaluated in general practice.4

Anemia may be present because of hemorrhage or concurrent immune-mediated hemolytic anemia (Evans' syndrome). About 20% of dogs with immune-mediated thrombocytopenia have concurrent immune-mediated hemolytic anemia.2 The presence of spherocytes or agglutination on a blood smear is helpful in diagnosing concurrent immune-mediated hemolytic anemia in an anemic patient with thrombocytopenia. Schistocytes may be present in patients with Evans' syndrome and concurrent disseminated intravascular coagulation, but their presence should also raise the index of suspicion for splenic hemangiosarcoma and can be seen with other conditions such as vasculitis.17

Leukocyte counts may be normal, or a stress leukogram may be present. In some patients with concurrent immune-mediated hemolytic anemia, a marked neutrophilia with a left shift may be present.2 Patients with immune-mediated thrombocytopenia generally have a less pronounced inflammatory leukogram; a marked neutrophilia, left shift, or toxic change should be evaluated for underlying inflammatory disease.


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Source: VETERINARY MEDICINE,
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