Platelets are blood cells that are essential for blood clotting. Platelet disorders can include inappropriate number of platelets (too many or too few), or normal number but inappropriate functioning of the platelets. Any platelet disorder affects blood clotting.

Disorders of platelet function can be caused by congenital diseases or acquired conditions. Acquired platelet function disorders are disorders of inappropriate platelet function, and that develop as a result of another disease or condition (acquired). In many cases, the platelet count may be normal or even elevated, but evidence of a bleeding disorder will be present. Myeloproliferative disorders such as primary thrombocytopenia, polycythemia vera, chronic myelogenous leukemia, and myelofibrosis can produce abnormalities in platelet function.

Other causes include renal failure, multiple myeloma, and medications such as aspirin and other anti-inflammatory medications, penicillins, phenothiazines, and prednisone (prolonged use).