What Is a Platelet Count (PLT)?

Platelets, or thrombocytes, are cells that form in the bone marrow. They perform the important function of clotting the blood when a blood vessel wall is damaged and bleeding. A simple blood test can determine if someone has a proper amount of platelets, or if they have too little (thrombocytopenia) or too many (thrombocytosis). 

This article discusses the platelet count (PLT) test in more detail.

Active blood platelets, illustration

SEBASTIAN KAULITZKI / SCIENCE PHOTO LIBRARY / Getty Images

What Are Platelets? 

Inside the bones of the body is the bone marrow, a spongy substance that produces all types of blood cells, including infection-fighting white blood cells, as well as red blood cells.

Platelets are also made in the bone marrow. When a blood vessel wall becomes injured, platelets respond to the injured area. They begin to clump together, or aggregate, to heal the blood vessel wall. The activated platelets also send out a signal to recruit more platelets to assist in the plugging of the blood vessel.

Platelets have an average lifespan of eight to 10 days.

Purpose of PLT Test

Making sure someone has the proper number of platelets can be very important to know, and a platelet count test may be ordered.

The PLT is the number of platelets present in a microliter of blood:

  • Thrombocytopenia: Too few platelets—could potentially lead to problems with easy bleeding or bruising
  • Thrombocytosis: Too many platelets—can lead to the development of abnormal clotting when not necessary

A healthcare provider may check a PLT as part of a routine healthcare checkup, or they may order it if they suspect someone has too many or too few platelets. 

How Is PLT Tested?

A PLT test can be ordered alone, but is often done in a larger blood test panel called a complete blood count (CBC), which tests for the numbers and sizes of white blood cells, red blood cells, and platelets. Both tests are completed by drawing a blood sample from a vein, most likely a vein in the arm. No special preparation is needed for a PLT test, and no fasting is required. 

Interpreting Results

The normal range of platelets is 150,000 to 450,000 per microliter of blood. If the result falls within this range, there is no abnormality in the platelet count. Having a number less than normal is called thrombocytopenia, and a level higher than 450,000 is called thrombocytosis.

Follow-Up

If the PLT test demonstrates someone has thrombocytosis or thrombocytopenia, following up with a healthcare provider is important. 

High Platelet Count

Thrombocytosis is when there are more than the normal number of platelets in the blood. There are two types of thrombocytosis: primary and secondary. 

Primary Thrombocytosis

With primary thrombocytosis, the bone marrow is functioning abnormally and making too many platelets than what is normal. Typically the platelets that are made by the bone marrow do not work properly and can lead to abnormal formation of blood clots, or even bleeding. Having blood clots can lead to the development of severe complications, such as a pulmonary embolism, heart attack, or stroke.

To treat primary thrombocytosis, medications can be given to reduce the number of platelets that are being made by the bone marrow. Examples of these medications include Hydrea (hydroxyurea) and Agrylin (anagrelide). Other medications that may be given include aspirin, which helps to keep the platelets from clumping together abnormally.

Secondary Thrombocytosis

In secondary thrombocytosis, the platelets are elevated as a reaction to something else that is going on. This can include:


Secondary thrombocytosis can resolve when the underlying disease or condition is treated. 

Low Platelet Count

Thrombocytopenia can be caused by a number of factors, including from decreased production of platelets in the bone marrow, abnormal destruction of the platelets by the immune system, or from the spleen holding onto the platelets.

Reasons for decreased platelet production in the bone marrow include:

Treating thrombocytopenia due to low platelet production in the bone marrow differs based on the cause. For example, treating cancer that is causing thrombocytopenia may bring the platelets back up to normal, or stopping a medication that is causing the low platelets can help bring their level up. 

Sometimes the body’s immune system destroys platelets inappropriately; this is referred to as immune thrombocytopenia. If platelets are mildly decreased, treatment may not be needed. In cases where platelets are too low, medications to suppress the immune system—such as steroids—can be given, or immunoglobulin infusions may be used. 

The spleen is normally an area in which some of the body’s platelets are stored. If the spleen is enlarged, it may hold onto more platelets than normal, causing platelet counts to be low. The spleen will release the platelets if they are needed though, and they generally still function properly. Sometimes a splenectomy, or surgical removal of the spleen, is required if platelet counts are too low. 

Frequently Asked Questions

What does a high PLT count mean? 

A high platelet count, or thrombocytosis, means there are more than 450,000 platelets per microliter of blood. Having too many platelets can be caused by bone marrow functioning abnormally and producing too many platelets. Thrombocytosis can also occur as a reactive process to iron deficiency, nutritional deficiency, or infection.

What does a low PLT count mean? 

A low platelet count means there are less platelets in the blood than normal, or less than 150,000 platelets per microliter of blood. Thrombocytopenia can be caused by bone marrow not working properly to make enough platelets, abnormal destruction of platelets by the immune system, or an enlarged spleen.

What is a normal platelet count? 

A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood.

What infections cause high platelet count? 

Platelets can increase as a result of many bacterial and viral infections.

A Word From Verywell

If you have concerns that your platelet count may be too low or too high, talk to your healthcare team. A simple blood test can determine if your platelets are in the right range.

If they are too low or too high, you may require some sort of treatment. Talking to your healthcare team can be an important way to make sure you stay healthy. 

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Article Sources
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  1. Johns Hopkins Medicine. What are platelets and why are they important?

  2. MedlinePlus. Platelet disorders.

  3. Sahler J, Grimshaw K, Spinelli SL, Refaai MA, Phipps RP, Blumberg N. Platelet storage and transfusions: new concerns associated with an old therapyDrug Discov Today Dis Mech. 2011;8(1-2):e9-e14. doi:10.1016/j.ddmec.2011.06.001

  4. National Heart, Lung, and Blood Institute. Thrombocythemia and thrombocytosis.

  5. National Heart, Lung, and Blood Institute. Thrombocytopenia.