Dr. Lauren Young, MD

When a patient has sufficient blood volume and lacks one or more components, a complete blood count (CBC) test can help determine if a patient requires a transfusion by measuring four blood components: red blood cells, white blood cells, platelets, and hemoglobin. If any of the patient's component counts is significantly below a standard range, the patient will likely need a blood transfusion. The complete blood count test uses a needle to extract a small amount of blood for testing.

Blood transfusion recipients should be tested for their blood type, which is one of four different types: type A, B, AB, or O. Patients with A, B, and O blood can only receive a blood donation matching their blood type or of O blood, which is known as the universal donor. Those with AB blood are considered "universal recipients"; they may receive A, B, AB, or O blood. Further complicating blood types is the presence or absence of the RhD antigen, adding a positive or negative modifier that should also be considered. Incompatible blood donations may cause transfusion sickness, such as a hemolytic reaction, where the body fights the donated red blood cells.

In certain emergency or trauma situations, medical teams often do not perform a complete blood count (CBC) test or blood type test. In those cases, patients are often given O negative blood, which is universally accepted. Some patients have their blood type on a medical card or bracelet, allowing emergency medical teams to issue compatible blood without testing.

Transfusions are performed by inserting a needle attached to an intravenous line (IV) into a vein in the body. This IV line is attached to a bag of blood that steadily dispenses blood into the patient's bloodstream. The type, volume, and components of donated blood depend on the patient's condition and needs. After the transfusion is complete, patients are generally permitted to resume normal activity, unless prohibited by a condition or injury. Some patients may experience bruising or soreness around the area that the needle was inserted.

At times, a second complete blood count test is performed to determine if the transfusion has brought patients' component counts to normal levels. Some patients may require multiple blood transfusions to reach a healthy blood volume and blood component count.

Once patients' blood volumes and components reach sufficient levels, they will generally begin to feel better. Patients of all ages, from newborn infants to elderly adults, can receive a blood transfusion.