This is a condition characterised by oversized platelets and a seemingly low platelet count. The name is derived from words macro (large) and thrombocyte (platelets).  The causative mutation was originally identified in the Cavalier King Charles Spaniel, and later a similar mutation was also discovered in affected Norfolk Terriers and Cairn Terriers.

Platelets, or thrombocytes, have a key role in bleeding prevention through clumping of blood vessel injuries. Although this condition caseues abnormalities in size of platelets. Platelets in affected dogs function normally, and affected dogs do not show any health problems or prolonged bleeding time. Some studies suggested other health issues such as heart disease may be related to large plateles, but more research is needed to confirm this theory.