Platelets are cell fragments that circulate in the bloodstream and help make blood clots (see Blood Clotting Process: How Blood Clots). Thrombopoietin, primarily produced in the liver, stimulates the bone marrow to make platelets. Platelets that are not used in clots circulate for 7 to 10 days and are then destroyed. About one third are always stored in the spleen. The platelet count (number of platelets circulating in the bloodstream) can decrease near the end of pregnancy (gestational thrombocytopenia) and increase in response to inflammation (secondary, or reactive, thrombocytosis).

Platelet disorders include

• An abnormal increase in platelets (essential thrombocythemia and reactive thrombocytosis)
• A decrease in platelets (thrombocytopenia)
• Platelet dysfunction

Any of these disorders can cause problems with blood clotting.

In essential thrombocythemia, an abnormal blood-forming cell in the bone marrow grows uncontrollably and produces too many platelets. The increased number of platelets most often causes excessive clotting, but some people develop bleeding. In reactive thrombocytosis, another disorder causes too many platelets to be produced. Such disorders include infections, chronic inflammation (such as occurs in rheumatoid arthritis and inflammatory bowel disease), iron deficiency, and certain cancers.

Last full review/revision December 2012 by David J. Kuter