Mean Platelet Volume (MPV)
Individual platelets can vary markedly in size within a given sample, especially in blood
from dogs and cats (even in health), so the presence of some large platelets may not be an abnormal finding. Large platelets have more clinical relevance in animals that are thrombocytopenic (see below).
The mean platelet volume is measured by the hematology analyzer in
a manner analogous to that described for the MCV for red blood cells.
Extreme variation in platelet size in a blood film from a normal dog.
- Artifact: Platelet clumping will falsely increase the MPV (a clump of platelets are viewed as a single large platelet by the analyzer). Platelet clumping will also falsely decrease the platelet count and, in some situations, may invalidate the count.
- Active thrombopoiesis: Analogous to immature anucleate erythrocytes, young platelets are frequently (but not always) larger than normal. In a thrombocytopenic animal, the presence of large platelets suggests that the low platelet count in blood is not due to decreased bone marrow production of platelets (however not all large platelets are young) and points to peripheral causes, e.g. consumption, destruction or sequestration, for the thrombocytopenia. Note that the absence of large platelets in a thrombocytopenic animal does not indicate that the bone marrow is not responding to the thrombocytopenia.
A reticulated platelet count can be performed to more accurately assess the bone marrow's response to a thrombocytopenia. Like immature anucleate erythrocytes, immature platelets have more RNA than normal, which can be detected and quantified using RNA-binding fluorescent dyes. This test is not frequently performed because it is not routinely available on most automated hematology analyzers.
- Abnormal platelet production: This can result in larger platelets than normal. Causes of abnormal platelet production include (but are not limited to) inherited conditions and primary myelodysplasia (a clonal disorder of hematopoiesis). An example of an inherited disorder of thrombopoiesis occurs in Cavalier King Charles Spaniels. These dogs can be quite thrombocytopenic (as low as 30,000/µL) and have very large platelets, but are clinically asymptomatic. The abnormal platelet production is due to a β-tubulin defect in megakaryocytes.
This is not a clinically relevant finding.
Copyright © Cornell University