Macrothrombocytosis in cavalier King Charles spaniels

Low platelet counts (< 150 x 10(9)/litre) have been measured by autoanalyser in cavalier King Charles spaniels with a wide range of diseases. However, the direct examination of blood smears revealed adequate numbers of platelets but many abnormally large platelets and it was not clear whether the large platelets were the result of disease or a breed abnormality. The numbers and diameters of the platelets in a group of clinically normal King Charles spaniels and a group of other breeds were measured by autoanalyser and manually. The automated platelet counts were lower in the King Charles spaniels than in the other breeds (P < 0.01), but the manual counts were similar. The platelet diameter was greater in the cavalier King Charles spaniels (median values 2.5 to 3.75 microns, P < 0.001) than the other breeds (median values 1.25 to 2.5 microns) and had a bimodal distribution with peaks at 1.25 to 2.5 microns and 3.75 to 5 microns. No clinical evidence of platelet dysfunction was detected. These findings indicate that cavalier King Charles spaniels may have large platelets which could lead to the erroneous diagnosis of thrombocytopenia by automated haematological analysis.