A crossmatch test is a blood test that is used to determine if a blood transfusion is compatible. It is performed by mixing the donor blood with the recipient's blood to see if there are any antibodies that will attack the donor blood. The crossmatch test is usually performed before a blood transfusion. It is also performed if the recipient has had a previous blood transfusion or if they have a condition that makes them more likely to have antibodies against donor blood. The crossmatch test is a simple and painless procedure that is usually performed in a doctor's office or a laboratory. The patient will need to have a blood sample drawn from a vein in their arm. The results of the test are usually available within a few hours. The crossmatch test is a safe procedure. However, there are some risks associated with any blood test, including: Bleeding at the needle site. Infection. Allergic reaction to the needle or the test solution. It is important to talk to your doctor about the risks and benefits of the crossmatch test before having the test. Here are some of the reasons why your doctor might order a crossmatch test: You are scheduled to have a blood transfusion. You have had a previous blood transfusion. You have a condition that makes you more likely to have antibodies against donor blood. If you have a crossmatch test, it is important to follow your doctor's instructions. This may include avoiding strenuous activity and drinking plenty of fluids.