A crossmatch is a medical test to confirm the compatibility of donor blood, tissue, or organs. A doctor will request this test after basic screening suggests that the donor material should be compatible. The goal is to identify any reactions that might make a transfusion or transplant dangerous. In an emergency situation, a doctor can request immediate donor material that matches a patient's type, but does not necessarily crossmatch. This usually occurs when the risk of death without the transfusion is greater than the risk of illness or injury because of it.
In a crossmatch procedure, a technician takes serum from the patient's blood and inoculates it with cells from the donor's blood. If the reaction is negative, nothing happens. The patient does not have antibodies to the donor's cells, or has them in such low concentrations that they do not trigger an immune response. If the sample starts to clump up, it indicates that antibodies are present and a transfusion or transplant cannot safely be performed because the patient's immune system will attack the donor material.
Many laypeople are familiar with the concept of matching by type, often using the ABO blood grouping. Patients with blood from different groups can experience reactions during transfusions as a result of antibodies. Type, however, is not the only measure of compatibility. A patient could receive a transfusion of blood that is theoretically a match and still experience a reaction because of other components in the blood known as factors. The crossmatch looks for factors that might cause a problem.
In addition to performing a physical crossmatch, it is also possible to do an electronic one. This relies on a detailed profile of the patient's blood. If a patient has a negative antibody screening, the computer can search for a match in a donor database. This type of crossmatch is not available in all settings but can be useful in a location like a blood bank for quickly identifying units of blood that should be safe for transfusion.
Transfusions and transplants ideally include a type and crossmatch before the recipient comes into contact with donor material. This is an important part of the screening for patient safety. Before donor blood even enters the pool of available material, it will also be rigorously screened for signs of infectious organisms and other issues that might make a transplant or transfusion dangerous. Blood from a patient with hepatitis C, for example, cannot be used in transfusions because it would infect the recipient.