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Individual platelets can vary markedly in size within a given sample, especially in blood from dogs and cats (even in health). The mean platelet volume is measured only be automated analyzers, such as the Advia. This measures mean platelet volume in a manner analogous to that described for the MCV for red cells.
Little is documented in the literature regarding the clinical interpretation of this parameter. In very general terms, increased MPV might be expected in "regenerative" thrombocytopenia, i.e., that caused by increased peripheral loss, destruction, or utilization of platelets and accompanied by increased production of platelets by marrow (megakaryocytic hyperplasia). Accelerated thrombopoiesis tends to result in the release of larger platelets (which also have enhanced functional capabilities). In apparent contradiction to this generality, one clinical study reported the frequent occurrence of decreased MPV in dogs with immune-mediated thrombocytopenia (due to platelet fragmentation), at least in the initial samples from newly-diagnosed cases.Recent studies in dogs demonstrate that a high MPV in dogs is a good predictor of an adequate bone marrow response, however a normal or low MPV in thrombocytopenic dogs does not predict an inadequate bone marrow response (in other words, in a thrombocytopenic animal, a large MPV indicates active thrombopoiesis, but a small or normal MPV does not rule out adequate numbers of megakaryocytes in the bone marrow). Healthy cats generally have variably sized platelets, some of them quite large. Some breeds of dogs, e.g. Cavalier King Charles Spaniels, have larger platelets than other breeds. These may be missed with some automated counters (such as the Coulter analyzers) because the platelets fall outside the size threshold the instrument uses for counting platelets. This does not occur with the Advia analyzer, which counts platelets of all sizes. Our experiences with this parameter have not shown it to be highly useful in distinguishing between various causes of thrombocytopenia in animals.
Last Updated: June 2000 |
Extreme variation in platelet size in a blood film from a normal dog.
