Plateletcrit is superior to platelet count for assessing platelet status in Cavalier King Charles Spaniels

Background: Many Cavalier King Charles Spaniel (CKCS) dogs are affected by an autosomal recessive dysplasia of platelets resulting in fewer but larger platelets. The IDEXX Vet Autoread (QBC) hematology analyzer directly measures the relative volume of platelets in a blood sample (plateletcrit). We hypothesized that CKCS both with and without hereditary macrothrombocytosis would have a normal plateletcrit and that the QBC results would better identify the total circulating volume of platelets in CKSC than methods directly enumerating platelet numbers. Objectives: The major purpose of this study was to compare the QBC platelet results with platelet counts from other automated and manual methods for evaluating platelet status in CKCS dogs. Methods: Platelet counts were determined in fresh EDTA blood from 27 adult CKCS dogs using the QBC, Sysmex XT-2000iV (optical and impedance), CELL-DYN 3500, blood smear estimate, and manual methods. Sysmex optical platelet counts were reanalyzed following gating to determine the number and percentage of normal- and large-sized platelets in each blood sample. Results: None of the 27 CKCS dogs had thrombocytopenia (defined as < 164 x 10(9) platelets/L) based on the QBC platelet count. Fourteen (52%) to 18 (66%) of the dogs had thrombocytopenia with other methods. The percentage of large platelets, as determined by regating the Sysmex optical platelet counts, ranged from 1% to 75%, in a gradual continuum. Conclusions: The QBC may be the best analyzer for assessing clinically relevant thrombocytopenia in CKCS dogs, because its platelet count is based on the plateletcrit, a measurement of platelet mass.