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Thrombocytopenia is an abnormally low number of platelets in the blood. Platelets are the blood cells responsible for helping blood to clot. Most thrombocytopenia causes no major problems, but severe thrombocytopenia may result in excessive bleeding. Many medical conditions and drugs can cause thrombocytopenia.

Platelets are very small blood cell fragments, a fraction of the size of a red blood cell. Platelets circulate in the bloodstream and gather together at sites of bleeding. There, platelets bind to each other, forming a plug that becomes a blood clot.

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There are normally at least 150,000 platelets present in a microliter of blood. Fewer than 150,000 platelets per microliter means thrombocytopenia is present.

The diagnosis of thrombocytopenia is made easily by a lab test called a complete blood count.

Causes of Thrombocytopenia

Thrombocytopenia can result from numerous medical conditions or as a side effect of medications. Generally speaking, thrombocytopenia results from either too few platelets being produced or existing platelets being destroyed.

Conditions causing destruction of existing platelets include:

  • Idiopathic thrombocytopenic purpura
  • Thrombotic thrombocytopenic purpura
  • Other autoimmune disorders, such as lupus and rheumatoid arthritis
  • Preeclampsia (HELLP syndrome) in pregnant women
  • Cardiac bypass surgery
  • Medication side effects (heparin, valproic acid and other anti-seizure drugs, quinidine, sulfa drugs, certain heart drugs, non-steroidal anti-inflammatory drugs, stomach drugs, and many others)

Conditions that reduce the body’s production of new platelets include:

  • Viral infections (chickenpox, parvovirus, hepatitis C, mononucleosis, HIV)
  • Blood cancers (leukemia, lymphoma)
  • Many chemotherapy drugs
  • Radiation therapy directed at bone marrow
  • Heavy alcohol use
  • Vitamin B12 or folate deficiency

Other causes of thrombocytopenia include:

  • Dilutional thrombocytopenia, in people receiving massive blood transfusions.
  • An enlarged spleen (splenomegaly), which can become filled with up to 90% of the body’s platelets.
  • Pregnancy, which causes thrombocytopenia in up to 5% of healthy women.

Newborn infants may have low platelet counts, a condition called neonatal thrombocytopenia. Neonatal thrombocytopenia is rare among healthy infants, but occurs often in newborns admitted to intensive care units.

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