Understanding Platelet Counts: What You Need to Know

Are you familiar with platelet counts and their significance in animal health? Platelets play a crucial role in the clotting process, preventing excessive bleeding. Understanding platelet numbers can help identify potential health issues in your beloved pets. In this article, we will delve into the world of platelet counts and provide insights to help you better understand their importance.

Critical Values: How Much Is Enough?

Platelet counts can vary across different species. In normal dogs and cats, platelet counts should be close to or exceed 200,000/µL. Horses, on the other hand, should have platelet counts of 100,000/µL or higher, with foals having even higher counts, usually exceeding 200,000/µL. Ruminants, such as cows and sheep, typically have higher platelet counts than horses, usually surpassing 400,000/µL, especially in younger animals. It’s important to note that spontaneous bleeding solely attributed to thrombocytopenia (low platelet counts) usually does not occur unless platelet counts drop below 30,000/µL. However, human guidelines indicate a cut-off of less than 10,000/µL for spontaneous bleeds, which is subject to change. Research on mice suggests that induced bleeding can persist with platelet counts below 25,000/µL. Interestingly, some animals with platelet counts as low as 10,000/µL may show minimal signs of bleeding. These animals often have enlarged platelets (with increased mean platelet volume), suggesting increased megakaryopoiesis, which enhances their functionality in hemostasis. Additionally, it is possible that inflammation-induced alterations in vascular integrity prompt bleeding in animals with low platelet counts. It’s worth noting that there are also guidelines for human patients with thrombocytopenia, providing recommendations on when to consider platelet transfusions.

Spontaneous hemorrhage should not be attributed solely to thrombocytopenia when platelet counts exceed 30,000/µL. Animals that have undergone traumatic injury or surgery may experience bleeding even at higher platelet counts, generally below 50,000/µL. In such cases, it’s essential to consider additional factors that could contribute to the occurrence of hemorrhage, such as abnormal platelet function or disorders of secondary or tertiary hemostasis. Bleeding caused by thrombocytopenia typically manifests as petechial or ecchymotic hemorrhages in the skin and mucous membranes. Other signs may include retinal hemorrhage, epistaxis, melena, hematochezia, and hematuria, primarily involving small blood vessels and mucosal surfaces. In severe cases of thrombocytopenia, intracavity hemorrhages, such as hemothorax or hemoabdomen, may occur.

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Sample Considerations: Ensuring Accuracy

When it comes to platelet counting, the appropriate anticoagulant to use is EDTA (purple-top tube). It is important to avoid using heparin as it can cause platelet clumping and yield inaccurate results. In rare instances, EDTA-dependent pseudothrombocytopenia can occur. This phenomenon arises from EDTA-induced exposure of platelet antigens, causing clumping in patients with antibodies that could be naturally occurring. Consequently, the platelet count appears artificially low, but platelet clumps are visible in a peripheral blood smear. To obtain an accurate count in such cases, blood collection should be done using citrate (blue-top tube) anticoagulant. However, it is crucial to account for the 10% decrease in platelet count due to dilution effects caused by citrate. Although EDTA-dependent pseudothrombocytopenia is a rare occurrence in horses, it has been recognized. It’s worth noting that platelet counts may decrease and mean platelet volumes may increase with sample storage, hence it is recommended to perform platelet counts as soon as possible after blood collection. Any clotting of the sample renders the platelet count invalid.

Method of Measurement: Different Approaches

There are various methods available for measuring platelet numbers accurately. The following are commonly used methods:

Smear Estimate

Assessing platelet numbers is part of the routine examination of blood smears. Severe thrombocytopenia is usually easily detected through this method. By examining a stained blood film, a fairly valid estimate of platelet count can be made if platelets are distributed singly and randomly throughout the smear. However, it is crucial to thoroughly examine the feathered edge of the smear for platelet clumps before estimating platelet counts. Platelet clumps are more common in samples collected from small peripheral veins, which may collapse during venipuncture, compared to larger veins like the cephalic or jugular. The presence of platelet clumps can decrease the accuracy of platelet counts obtained by any method and may even invalidate the count. Cats, in particular, have platelets that are highly reactive and prone to clumping. Therefore, obtaining accurate platelet counts in cats can be challenging. In the monolayer of the smear, each platelet seen under the 100x oil immersion objective is roughly equivalent to 15,000 platelets/µL. Therefore, if there are more than 10 to 15 platelets per field on average, the platelet count is within or above the reference interval.

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Manual Count

For more accurate quantification of platelets, a manual count can be performed using an Unopette system and counting platelets in a hemacytometer.

Automated Counts

Automated hematology analyzers or electronic impedance counters are commonly used for platelet counting. However, they may underestimate the platelet count in cases where platelets are unusually large or when platelet clumps are present. Therefore, it is important to verify the platelet count obtained, whether through manual or electronic methods, by estimating platelet numbers from a blood smear and checking for clumps. It’s worth noting that warming blood does not minimize platelet clumping or significantly increase platelet clumps in dogs, cats, or horses.

Test Interpretation: What the Numbers Reveal

Decreased Number (Thrombocytopenia)

Thrombocytopenia, characterized by a decreased platelet count, can be caused by various pathophysiological mechanisms, including:

  • Decreased production of platelets in the bone marrow
  • Increased consumption or use of platelets in coagulation
  • Increased destruction or clearance of platelets by macrophages or the liver
  • Sequestration of platelets in the spleen or microvasculature

Increased Number (Thrombocytosis)

Thrombocytosis, on the other hand, refers to an increased platelet count. It can occur as a primary myeloproliferative condition or as a secondary (reactive) phenomenon in various physiological and pathological states. Reactive thrombocytosis, usually indicated by a platelet count exceeding the reference interval for the species, is typically not of direct pathological importance. Young animals, especially calves and foals, naturally have platelet counts higher than the adult reference interval.

For more in-depth discussion on thrombocytopenia and thrombocytosis, refer to the disorders of platelet number section.

Now that you have a better understanding of platelet counts and their significance, you can be more informed about your pet’s health. Keep an eye out for any abnormal platelet counts and consult with your veterinarian if necessary. Remember, platelets are an essential component of your pet’s well-being, ensuring their overall health and happiness.

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