Platelet counts

Normal dogs and cats should have platelet counts close to or >200,000/Ál; normal horses and cows should have platelet counts of 100,000/Ál or greater. Bleeding, that can attributed solely to thrombocytopenia, does not occur unless platelet counts are < 30,000/Ál. Some animals with platelet counts as low as 10,000/Ál have minimal signs of bleeding. Many of these latter animals have large platelets (with increased MPVs, indicating increased megakaryopoiesis), that are thought to be more functional in hemostasis. Hemorrhage should not be ascribed totally to thrombocytopenia at platelet counts > 30,000/Ál. If an animal is showing clinical signs of hemorrhage with counts above this value, additional defects (such as abnormal platelet function or disorders of secondary or tertiary hemostasis) should be considered as contributing to the hemorrhage . Bleeding due to thrombocytopenia usually is manifested as petechial or ecchymotic hemorrhages in the skin and mucous membranes, retinal hemorrhage, epistaxis, melena, hematochezia, and/or hematuria; in other words, bleeding from small vessels. Severe thrombocytopenia can result in intracavity hemorrhage (hemothorax, hemoabdomen).

The anticoagulant required for platelet counting is EDTA (purple-top tube). Heparin causes clumping of platelets and is not recommended. In rare situations, an EDTA-dependent pseudothrombocytopenia can occur. This is due to EDTA-induced exposure of platelet antigens, which causes clumping in patients with antibodies (which can be naturally occurring) to these antigens. The result is a low platelet count that is artifactual, but this should be recognized as platelet clumps will be seen in a peripheral blood smear. The only way to get an accurate count in these patients is to collect blood into citrate (blue-top tube) anticoagulant (remembering that the citrate will decrease the platelet count by 10% due to dilution effects; this effect should be compensated for). EDTA-dependent pseudothrombocytopenia has been recognized in horses, but is a rare occurrence. Platelet counts will also decrease with storage (and MPVs increase), therefore counts should be performed as soon as possible after blood collection. Note that clotting of the sample will totally invalidate the platelet count.

Platelet number can be determined in several ways.

  • Estimate from a smear: A fairly valid estimate of platelet number can be made from examination of a stained blood film if platelets are distributed singly and randomly throughout the smear. Before estimating platelet counts from a blood smear, the feathered edge of the smear should be examined thoroughly for platelet clumps. Platelet clumps (see image below) occur readily if there is difficult venipuncture and slow or excessively turbulent blood flow during venipuncture. Platelet clumps are more numerous in samples collected from small peripheral veins (which can collapse during venipuncture), rather than larger veins like the cephalic or jugular. The presence of platelet clumps (at the feathered edge or throughout the smear) will decrease a platelet count obtained by any method (estimation, electronic or manual) and may even invalidate the count. Cats have notoriously reactive platelets, which clump readily on sample collection. Therefore, obtaining a platelet count can be difficult in a cat.
    Each platelet seen in a microscope field with the 100x oil immersion objective, in the monolayer of the smear, is roughly equivalent to 15,000 platelets/Ál. Therefore, if there are more than 10 to 15 platelets per field on average, the platelet count is within or above the reference range.
    Assessment of platelet number is part of the routine examination of blood smears; severe thrombocytopenia should be readily recognized by examination of a blood smear.

  • Quantitative counts: More accurate quantitation of platelets is done either by using a Unopette system and counting platelets in a hemacytometer (manual counts) or by counting platelets with an electronic cell counter. Electronic counters will underestimate the platelet count if platelets are very large (occurs in some cats and dogs, such as Cavalier King Charles Spaniels) or if there are clumps. The platelet count (whether obtained manually or electronically) should always be verified by estimating platelet numbers from a blood smear (with checking for clumps).