Abstract

Cavalier King Charles spaniel plateletcrit Harold Tvedten Cavalier King Charles spaniels (CKCS) often have an autosomal recessive dysplasia of their platelets (macrothrombocytosis) which results in fewer but larger platelets. The affected dogs of this breed have no signs of bleeding and the main problem is errors that veterinarians make in interpreting the low platelet count. The errors include additional unnecessary testing and even inappropriate treatment of some other type of thrombocytopenia. We evaluated the platelets of 27 CKCS and showed they had normal blood platelet mass (plateletcrit) despite lower platelet counts than canine reference values. Thus Plateletcrit is the test of choice for evaluating CKCS for detecting patients with true thrombocytopenia (not macrothrombocytosis). The IDEXX Vet Autoread hematology instrument (QBC) measures directly plateletcrit in a centrifuged tube and was the best method of 5 or 6 methods tested to measure plateletcrit. Sysmex XT-2000 iV optical platelet counts and manual platelet counts were most accurate in terms of enumerating the number of platelets in CKCS blood, but all methods besides the QBC gave results that would continue to cause confusion for veterinarians unaware of this hereditary dysplasia. Impedance platelet counts were low (< 164 x 109/L) in 59-63 % of the dogs. Optical Sysmex platelet counts were low in 52 % of the dogs. Manual platelet counts were low in 69 % of the dogs. All QBC platelet counts were normal. This statement requires explaining. The QBC measures the blood's platelet mass (percentage of total volume determined by width of the platelet layer) and converts this to a platelet count. This number is inaccurate in affected CKCS, but is clinical useful to show the dogs do not a deficiency of platelet mass. The circulating mass of platelets is what is functionally important in hemostasis and should detect true thrombocytopenia of other causes in CKCS. The QBC platelet count, though not accurate, indicates the dogs were normal by using as a "scale" veterinarians know how to interpret. Thrombocytopenia by definition in the medical dictionaries is a reduced platelet count and few or no veterinarians even look at plateletcrit results even if their instrument reports it. Platelets are universally interpreted as a count (platelets x 109/L). Thus the QBC platelet result has been reported as a "count" which is a calculated number. That number is not accurate but allows interpreting if the plateletcrit is normal or not. Plateletcrit should really be reported as a percentage of blood volume or l/l like the units for hematocrit. CKCS have a normal plateletcrit when affected with hereditary macrothrombocytosis or not. Thus an accurate test of plateletcrit should be used to test CKCS for clinically symptomatic or important causes of thrombocytopenia. Currently only the QBC has been shown to measure platelet mass in this breed. Additional information in the study included that percentage of large platelets in affected CKCS increased in a gradual continuum throughout the 27 dogs. There was no obvious border or threshold where affected dogs could be separated from unaffected dogs. An algorithm for identifying large platelets was developed on the Sysmex XT 2000 iV optical platelet count for excluding large platelets from the original platelet count. These larger platelets contained more RNA and this algorithm seems similar to the "immature platelet fraction" used on another Sysmex system to identify younger, more immature platelets. The CKCS large platelets are likely not young platelets but just big platelets and thus are at least 1 exception to the concept that large RNA rich platelets (so called reticulated platelets) are evidence for increased or active megakaryopoiesis. Platelet estimates from the blood smear gave similar results to the automated methods. It was additionally shown the CKCS with large platelets, had obviously more platelets at the feathered edge of the blood smear than CKCS with normal sized platelets. This supports the concept that larger cells get pushed out to the end of a smear and are unevenly distributed

Published in

Conference

European Society of Veterinary Clinical Pathology

SLU Authors

• Tvedten, Harold

  • Department of Clinical Sciences, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Animal and Dairy Science
Veterinary Science


Permanent link to this page (URI)

https://res.slu.se/id/publ/18797