What Are Platelets?

Blood is made up of a liquid called plasma and three main types of blood cell:

• Red blood cells (also known as “erythrocytes”) carry oxygen around the body.
• There are several different types of white blood cells (also known as “leukocytes”) and they help fight infection.
• Platelets (also known as “thrombocytes”) are the smallest type of blood cell and are involved in the process that enables blood to clot to promote healing and control “hemostasis” (the process that stops us from losing blood if we begin to bleed).

How Do Platelets Help Control Blood Loss?

The process by which blood clots are formed involves platelets and a number of different types of protein. Platelets are made in the bone marrow and continuously travel around the body in the bloodstream. Under normal conditions, when there is no damage to blood vessels that needs to be repaired, platelets are “quiescent” (inactive) and plate-shaped. If a blood vessel becomes damaged, platelets start to be attracted to the site of injury because a protein called collagen becomes exposed. Once they get there, they start to stick to the collagen and also to each other. This is helped by the cells lining the damaged blood vessel releasing a molecule called von Willebrand factor.

The platelets then become activated. As this happens, their shape changes to spherical with long “spines” or “tentacles”. The platelets start to secrete chemical signals that attract other platelets to the injury site, and they clump together to temporarily cover and close the wound, a bit like a plaster. This platelet covering isn’t able to last long so clotting factors in the blood start to convert a protein called fibrinogen into a different form called fibrin. Fibrin can form long, tough, insoluble strands that bind to the platelets and cross-link together to form a strong, long-lasting mesh on top of the platelet plug. The fibrin then acts as a scaffold as part of the healing process.

What Is Platelet Transfusion and When Is It Needed?

Platelet transfusion is the process by which platelets that have been donated are transferred into the bloodstream of another person. The body needs to have a certain number of platelets in the blood to be able to control hemostasis properly. Too few or too many platelets can cause problems. A normal platelet count is considered to be 150,000–450,000 platelets per microliter of blood. If a person’s platelet count is greater than this they are classed as having a condition called thrombocytosis. Thrombocytosis can have a number of different causes, which affect how serious it is and whether or not a person needs treatment. If a person’s platelet count is less than 150,000 platelets per microliter of blood they are classed as having thrombocytopenia.

Thrombocytopenia can develop as the result of a number of conditions, including cancer, some types of anemia, autoimmune conditions, and viral infections, and as a result of certain types of medical treatment. Symptoms can include frequent gastrointestinal bleeding or bleeding from the gums and nose, and bruising easily. A person may also have a low platelet count if they have bled severely (for example during surgery or as the result of being in an accident) or if their spleen (the organ that “cleans” the blood to keep it healthy) starts to remove too many platelets. In addition to having too few platelets, people may also need a platelet transfusion if they have a platelet function disorder that means that they have enough or too few platelets but they do not work properly.

What Happens to Donated Platelets?

There is a constant and increasing demand for donations of platelets. Some donations are used to transfuse people with a low platelet count, while others are used to help patients who are receiving cancer treatment or are in intensive care. One of the reasons for the increasing need for new donors is that platelets don’t last very long. They are only usable for 7 days after they have been donated, and in the body they are removed from the bloodstream by the spleen or liver after 7–10 days.

Until the 1960s, platelets were stored in the cold (at 4 °C, i.e. in a fridge) because cold-stored platelets are better at stopping blood loss than ones stored at room temperature. However, research showed that the recovery of patients after platelet transfusion was better and that platelets lasted longer if they were stored at room temperature, and cold storage of platelets stopped. While this improved patient recovery after transfusion it brought a new problem; storage of platelets at room temperature increases the risk of septic transfusion reactions caused by the platelets being contaminated with bacteria.

What Is Transfusion-Transmitted Sepsis?

Transfusion-transmitted sepsis can develop if a patient is transfused with donated platelets that are contaminated with bacteria. It is typically caused by contamination with bacteria that usually live harmlessly on a person’s skin, contamination getting into the platelet sample during collection or processing, or the donor unknowingly having bacteria in their blood. Symptoms can begin during or shortly after a transfusion and include severe shivering and chills, high fever, nausea and vomiting, breathing difficulties, low blood pressure, a fast heart rate and circulatory collapse. Severe cases can be fatal.

What Did This Review Article Investigate?

Research continues to investigate ways to improve the safety of platelet transfusion and to optimize how platelets are stored. As a result, over recent years, interest in cold storing platelets has increased because it reduces the risk of bacterial contamination and potentially increases the length of time that platelets can be stored.

There is some evidence that storing platelets at room temperature for 5 days (during which time they are constantly gently shaken), followed by cold storage without shaking for up to another 16 days, may result in platelets being of better quality at the time of transfusion. Other studies have reported that cold-stored platelets are “primed” for activation to a greater extent when compared with platelets stored at room temperature, and also seem to be better at sticking to collagen at sites of blood vessel damage. In addition, one research group has reported that cold-stored platelets are able to form denser clots, with thinner fibers and more crosslinks, making them more effective at stopping blood loss.

Although these results seem promising, there is also evidence from research reports that how platelets are prepared for transfusion, the solutions that are added to them (such as plasma), and variations in how they are stored can affect how well they function. Until these factors become globally standardized it is difficult to draw conclusions regarding whether storage at room temperature or 4 °C is best. In the meantime, platelet donations are increasingly needed to help a broad group of patients with a variety of conditions. Requirements vary according to regions, but if you are interested in becoming a platelet donor you will be able to get information from the health service in your country.

Note: The author of this paper make a declaration about grants, research support, consulting fees, lecture fees, etc. received from pharmaceutical companies. It is normal for authors to declare this in case it might be perceived as a conflict of interest. For more detail, see the Conflict of Interest Statement at the end of the paper.