INTRODUCTION

Thrombocytopenia is a common finding in critically ill veterinary patients, regardless of the diagnosis at admission. There are four primary causes of thrombocytopenia: hypoproliferation (lack of production), sequestration, consumption (utilization), and destruction. Sampling or laboratory artifact may also lead to falsely low platelet counts. The critical care veterinarian’s job is to diagnose the cause of the thrombocytopenia, determine whether the platelet count is low enough to warrant intervention, and prescribe appropriate treatment. For the purpose of clarity and uniformity, thrombocytopenia will be defined here as a platelet count below the lower end of the reference range for the testing laboratory, although typical platelet numbers in dogs and cats should range from 200,000 to 800,000 cells/μl. However, certain breeds may normally have lower platelet counts in the absence of disease (i.e., Cavalier King Charles Spaniel and Greyhound).

BACKGROUND

Platelets are produced in the bone marrow from pluripotential stem cells under the influence of multiple colony-stimulating factors (CSF), interleukins, and hormones. In 1994, a gene encoding a protein that stimulates platelet production was cloned. The protein is called thrombopoietin. Produced in the liver, kidneys, brain, and testes, thrombopoietin is released constitutively and has several roles during platelet development. In vitro it increases megakaryocyte size and number and stimulates expression of platelet-specific cell surface markers. Because platelets are responsible for binding and removing thrombopoietin, thrombocytopenia leads to increased circulating levels of thrombopoietin, and therefore greater bone marrow stimulation. Conversely, thrombocytosis results in reduced circulating thrombopoietin and, subsequently, less bone marrow megakaryocyte production.¹

Platelets are not only important for hemostasis, but they also have a role in the inflammatory process and response to infection in normal humans and animals. The granules of platelets contain chemokines, growth factors, histamine, and serotonin. Platelets also are able to generate eicosanoids that mediate inflammation and hemostasis. Inflammation or contact with infectious agents can stimulate aggregation and degranulation of platelets. Thus platelet function in sepsis and critical illness can be both beneficial and detrimental to the patient.^2-5

Manual evaluation of the blood smear is an effective means of estimating platelet numbers before receiving an automated count (see Chapter 122, Blood Film Evaluation). Platelet counts are easily and rapidly estimated by evaluating the blood smear. There are normally 8 to 15 platelets per 100× oil-immersion monolayer field. This correlates with circulating platelet numbers between 200,000 and 800,000 cells/μl. If platelets are clumped on the blood smear, the distribution is uneven so that a manual estimate is inaccurate. Accordingly, the automated count will also be inaccurate. Cat platelets tend to clump together, leading to falsely low reported platelet counts. Overlap of cell size between feline red blood cells and platelets can also lead to falsely reduced platelet numbers when automated cell counters are used.⁶ When thrombocytopenia is reported in a feline patient, a blood smear should be evaluated (ideally by a clinical pathologist to confirm the result).