Platelets (thrombocytes) are tiny fragments of very large cells called megakaryocytes in the bone marrow. Platelets are released into the blood to circulate and are essential for proper blood clotting.
When there is an injury to a blood vessel or to tissue and bleeding begins, platelets help stop bleeding in multiple ways. They:
- Adhere to the injury site
- Clump together (aggregate) with other platelets to form a temporary plug
- Release chemical signals that stimulate further aggregation of other platelets
- Support the coagulation cascade, a process in which clotting factors in the blood are sequentially activated to help form a blood clot (thrombus)
- Eventually become part of a stable blood clot at the site of the injury that remains in place until the injury heals
There are normally about 150,000 to 450,000 platelets per microliter of blood. For reference, a drop of liquid is about 50 microliters. So, a healthy person will have millions of platelets in a single drop of blood. Because of many small injuries that occur regularly, platelets are constantly being used up. They have a life span of about 8 to 10 days. Thus, the bone marrow must continually produce new platelets to replace those that are degraded, used up, and lost through bleeding.
The number of platelets in your blood can be determined with a platelet count, a common test that is usually done as part of a complete blood count (CBC). You may have a CBC done as part of a routine health exam or when your healthcare practitioner is evaluating you for a particular condition.
Your platelet count is interpreted by your healthcare practitioner within the context of other tests that you have had done (e.g., CBC) as well as other factors, such as your medical history. If you have a low platelet count, it means you have too few platelets circulating in your blood. Your blood may not clot appropriately and you may be at an increased risk for easy bruising and/or excessive bleeding.
However, a single low platelet count may or may not have medical significance. Generally, this is the case when the result is only slightly lower than the reference (normal) range. Your healthcare practitioner may repeat the test and may look at results from prior platelet counts. On the other hand, a result outside the reference range may indicate a problem and warrant further investigation. Your healthcare practitioner will determine whether a result that falls outside of the reference range means something significant for you.
A low platelet count, also called thrombocytopenia, can be caused by various conditions. It may develop suddenly (acute), may be temporary, or persist over time (chronic).
The risk for serious bleeding with thrombocytopenia does not happen until the number of platelets is very low, around 20,000 per microliter or lower. However, minor bleeding can be seen at about 50,000 per microliter if another condition is present (e.g., kidney disease). The number of platelets can drop rapidly depending on the condition that is causing the low number of platelets.
If your platelets are severely low and you are at risk of bleeding, you may be treated to increase your platelets and to reduce bleeding risk. After that, testing may be done to help determine a cause.