Platelets (sometimes called thrombocytes) are cell fragments that circulate in the bloodstream and help blood to clot. Thrombopoietin, primarily produced in the liver, stimulates the bone marrow to make large cells (megakaryocytes), which in turn make platelets from their cytoplasm. 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) is usually about 140,000 to 440,000 platelets per microliter (140 × 109 to 440 × 109 per liter). The platelet count can vary according to the menstrual cycle. It can decrease near the end of pregnancy (gestational thrombocytopenia) and increase in response to inflammation (secondary, or reactive, thrombocytosis). Neither of these conditions is serious, and most affected people have no problems resulting from either one.
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, the bone marrow cells that make platelets grow excessively and make too many platelets despite no other disorder being identified. Surprisingly, the increased number of platelets most often causes excessive bleeding rather than clotting. Doctors sometimes give people aspirin to lower the risk of abnormal clotting if they have vascular disease or are at increased risk of a heart attack. Drugs to lower the platelet count may sometimes be needed.
In reactive thrombocytosis, another medical disorder stimulates the bone marrow to make too many platelets (the platelets are made in reaction to the other disorder). Such disorders include infections, chronic inflammation (such as occurs in rheumatoid arthritis and inflammatory bowel disease), iron deficiency, and certain cancers. The increased numbers of platelets usually do not cause any increase in clotting or bleeding risk. Specific treatment is not needed for the high number of platelets, but the underlying condition may need to be treated.
In thrombocytopenia, there are many causes of decreased numbers of platelets. Causes are generally divided into those involving decreased production of platelets and those involving increased destruction or loss of platelets.
In platelet dysfunction, people have the correct number of platelets, but the platelets do not function normally.
Bleeding in the skin may be the first sign of a low platelet count or platelet dysfunction. Many tiny red dots (petechiae) often appear in the skin on the lower legs, and minor injuries (including needlesticks) may cause black-and-blue bruises (ecchymoses or purpura). The gums may bleed, and blood may appear in the stool or urine. Menstrual periods or nosebleeds may be unusually heavy. The lower the platelet count, the more severe the symptoms.
People with too many platelets may have the same signs of bleeding as people with too few platelets because a very high number of platelets can absorb enough of an important clotting factor (a protein found in blood that works with platelets to help the blood clot ) to cause bleeding. However, some people instead have symptoms caused by abnormal blood clots. For example, people may have a swollen leg from a blood clot in a vein in their leg, or they may have numbness and weakness because of a blood clot in their brain causing a stroke.
- Complete blood count
Doctors can diagnose many platelet disorders by a simple blood test (a complete blood count [CBC]) to measure the number of platelets.
Special tests may be needed to diagnose platelet dysfunction.