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Thrombocytopenia: causes of low platelets, and high counts
Thrombocytopenia in a dog or cat is a platelet count below the lab's interval, and the first job is to rule out platelet clumping on a blood smear before reading it as immune destruction, consumption, an underlying inflammatory, infectious or neoplastic disease, or reduced marrow production.
Platelet counts are the CBC value most often wrong for reasons that have nothing to do with the patient. Clumping in EDTA is a common preanalytic error in dogs and cats, so every low count starts at the microscope1. Once the count is confirmed, the size of the drop and the company it keeps narrow the list quickly.
Platelet count
- High can mean
- Thrombocytosis. In 165 dogs it was secondary in every case, most often to neoplasia or inflammation, with endocrine disease less common; many of the dogs had received glucocorticoids or vincristine; marked counts were more likely with neoplasia, and thromboembolism occurred in a small number2.
- Low can mean
- Thrombocytopenia. In 871 thrombocytopenic dogs, inflammatory or infectious disease was the largest category, then neoplasia, miscellaneous disorders, disseminated intravascular coagulation and immune-mediated thrombocytopenia; dogs with immune-mediated disease and with DIC had the lowest counts3. Immune thrombocytopenia is the most common acquired primary hemostatic disorder in dogs, is less common in cats, and remains a diagnosis of exclusion; secondary forms arise from an underlying trigger4.
- Can shift it artificially
- EDTA-dependent clumping causes pseudothrombocytopenia. It should be considered in a dog with a moderate drop and no bleeding; a clean, nontraumatic venipuncture is often enough, and rare dogs need a sample in citrate5. Warming EDTA samples raised counts in dogs and cats but did not abolish clumping in every case1, and adding amikacin after collection broke up clumps and raised measured counts, most markedly in cats6. Cavalier King Charles spaniels can carry an inherited macrothrombocytopenia, and analyzers that size platelets may undercount the large ones, so a low count in this breed with no bleeding is read from the smear.
- Next test
- Smear review for clumps and a platelet estimate; then, for a confirmed low count, the trigger screen the ACVIM consensus lays out, including regional infectious disease testing and imaging for neoplasia4.
Immature platelet fraction
- High can mean
- Peripheral thrombocytopenia, where the marrow is releasing young platelets in response to destruction or consumption7.
- Low can mean
- Central thrombocytopenia from reduced marrow production, once pseudothrombocytopenia has been excluded on the smear7.
- Can shift it artificially
- Clumps. The study that validated the measure insists the smear is reviewed first7.
- Next test
- Where the analyzer reports it, the fraction can steer the work-up and help decide whether a bone marrow sample is needed7.
Clumps, cats and the smear
Cats deserve their own paragraph. In the warming study the median rise in count after warming was larger in cats than in dogs, and in the amikacin study the platelet count in clumped feline samples more than doubled after the additive1,6. A low analyzer count in a cat without bleeding is a smear question first. Look at the feathered edge for clumps, estimate the count from the monolayer, and repeat the draw with a clean stick if clumps are there5.
Bleeding tells you how far the count has fallen. Petechiae, ecchymoses or mucosal bleeding with a confirmed severe drop pull immune thrombocytopenia and DIC toward the top of the list, because those two categories had the lowest counts in the large canine series3.
Record what you did to the sample. A count repeated from a clean jugular draw, a citrate tube or a warmed tube is only comparable with the first result if the report says which was used, and a clump-free smear estimate is worth writing into the record beside the analyzer number. The consensus algorithm for immune thrombocytopenia starts from a confirmed count for the same reason: every later decision, from infectious disease testing to a marrow sample, rests on that first number being real4.
Reading these values together
A severe, confirmed thrombocytopenia with an otherwise unremarkable CBC and a well patient leans immune-mediated, but the diagnosis still requires the comorbidity screen, because secondary immune thrombocytopenia is driven by an underlying disease4. A moderate drop in a sick patient with a long problem list leans inflammatory, infectious or neoplastic, the categories that dominated the canine series3. Thrombocytopenia with prolonged clotting times points to consumption in DIC3. A high count in a patient on glucocorticoids or vincristine may be drug-associated; a marked high count without that history moves neoplasia up the list2.
Example: a panel worked through
Example. A 7-year-old neutered male mixed-breed dog has petechiae on the ventral abdomen. The analyzer platelet count is far below the lab's interval, the smear shows very few platelets and no clumps, the PCV is within its interval and the dog is otherwise bright.
The count is real, and with bleeding at this degree of drop, immune thrombocytopenia and DIC lead the list3. Coagulation times separate the two. If they are normal, the consensus trigger screen comes next: drug and vaccine history, infectious disease testing chosen for the region and travel history, and thoracic and abdominal imaging for neoplasia4.
Frequently asked questions
What causes low platelets in a dog?
In a large canine series, inflammatory or infectious disease and neoplasia were the commonest causes, followed by miscellaneous disorders, DIC and immune-mediated thrombocytopenia. Clumping in the tube must be ruled out first3,5.
Is a low platelet count in a cat usually real?
Often it is not. Clumping is a common preanalytic error in cats, so a smear review and, if needed, a clean repeat draw come before any work-up1,6.
What does a high platelet count mean in a dog?
Thrombocytosis in dogs is usually secondary, most often to neoplasia, inflammation or glucocorticoid or vincristine administration2.
Is there a single test for immune thrombocytopenia?
No. It is a diagnosis of exclusion, made after the comorbidity screen the ACVIM consensus describes4.
Sources
- Williams TL, Archer J. Effect of prewarming EDTA blood samples to 37°C on platelet count measured by Sysmex XT-2000iV in dogs, cats, and horses. Vet Clin Pathol. 2016; 45(3):444-9. PMID: 27391303. DOI: 10.1111/vcp.12378.
- Neel JA, Snyder L, Grindem CB. Thrombocytosis: a retrospective study of 165 dogs. Vet Clin Pathol. 2012; 41(2):216-22. PMID: 22390566. DOI: 10.1111/j.1939-165X.2012.00416.x.
- Botsch V, Küchenhoff H, Hartmann K, et al. Retrospective study of 871 dogs with thrombocytopenia. Vet Rec. 2009; 164(21):647-51. PMID: 19465753. DOI: 10.1136/vr.164.21.647.
- LeVine DN, Kidd L, Garden OA, et al. ACVIM consensus statement on the diagnosis of immune thrombocytopenia in dogs and cats. J Vet Intern Med. 2024; 38(4):1958-1981. PMID: 38752421. DOI: 10.1111/jvim.16996.
- Wills TB, Wardrop KJ. Pseudothrombocytopenia secondary to the effects of EDTA in a dog. J Am Anim Hosp Assoc. 2008; 44(2):95-7. PMID: 18316447. DOI: 10.5326/0440095.
- Vasilatis DM, Walker NJ, Borjesson DL. Amikacin disaggregates platelet clumps in EDTA blood samples from cats and dogs when added postcollection. Vet Clin Pathol. 2023; 52(2):228-235. PMID: 36849708. DOI: 10.1111/vcp.13203.
- Perez-Ecija A, Martinez C, Fernandez-Castañer J, et al. Utility of immature platelet fraction in the Sysmex XN-1000V for the differential diagnosis of central and peripheral thrombocytopenia in dogs and cats. J Vet Intern Med. 2024; 38(3):1512-1519. PMID: 38619127. DOI: 10.1111/jvim.17074.