A platelet count is often ordered as a standard part of a
complete blood count, which may be done as part of an annual physical examination. It is almost always ordered when a patient has unexplained bruises or takes what appears to be an unusually long time to stop bleeding from a small cut or wound.
In an adult, a normal count is about 150,000 to 450,000 platelets per microliter (x 10–6/Liter) of blood.
If platelet levels fall below 20,000 per microliter, spontaneous bleeding may occur and is considered a life-threatening risk. Patients who have a bone marrow disease, such as leukemia or other cancer in the bone marrow, often experience excessive bleeding due to a significantly decreased number of platelets (thrombocytopenia). Low number of platelets may occur in some patients with long-term bleeding problems (e.g., chronic bleeding stomach ulcers), thus reducing the supply of platelets. Individuals with an autoimmune disorder (such as lupus or Idiopathic Thrombocytopenia Purpura [ITP], where the body’s immune system creates antibodies that attack its own organs) can cause the destruction of platelets. Certain drugs, such as heparin, quinidine, sulfa drugs, oral anti-diabetic drugs, and even alcohol, may cause decreased platelet counts. Patients undergoing chemotherapy may also have a decreased platelet count. Up to 5% of pregnant women may experience thrombocytopenia at term.
More commonly (up to 1% of the population), easy bruising or bleeding may be due to an inherited disease called von Willebrand’s disease. While the platelets may be normal in number, their ability to stick together is impaired due to a decrease in von Willebrand’s factor, a protein needed to initiate the clotting process. Many cases go undiagnosed due to the mild nature of the disease; however, the more severe form can be devastating.
Increased platelet counts (thrombocytosis) may be seen in individuals who show no significant medical problems, while others may have a more significant blood problem called myeloproliferative disorder (abnormal growth of blood cell elements). Some may have a tendency to bleed due to the lack of stickiness of the platelets, yet in others, the platelets retain their stickiness but, because they are increased in number, tend to stick to each other, forming a clump that can get stuck within a blood vessel and cause damage, including death (thromboembolism).
Living in high altitudes may cause increased platelet levels, as can strenuous exercise.
Decreased levels may be seen in women before menstruation.
Drugs that may cause increased platelet levels include estrogen and oral contraceptives.
Other inherited disorders caused by defective platelets or decreased/absent proteins that activate the platelets include Glanzmann’s Throbasthenia, Bernard-Soulier disease, Chediak-Higashi syndrome, Wiskott-Aldrich syndrome, May-Hegglin syndrome, and Down syndrome. The occurrence of these abnormalities, however, is relatively rare.