Macrothrombocytopenia is inherited as an autosomal dominant trait.
The disease is characterised by low platelet count and larger than normal platelets.
Over half of all cavalier King Charles spaniels may have both an abnormally low number of platelets and also have oversized platelets in their blood. This benign condition is called idiopathic asymptomatic thrombocytopenia and is normal in cavaliers, requiring no treatment.
Testing is available. Animals are characterised as clear, affected heterozygous or affected homozygous. Affected homozygous animals are more severely affected than affected heterozygous animals.