Understanding Your Blood Typing and Compatibility Tests

Section Editors: Christopher McCudden Ph.D., DABCC, FADLM, FCACB and Kamran Mirza MD PhD
July 28, 2026

Blood typing and compatibility tests are a group of blood tests done to make sure that any blood a person receives in a transfusion is safe, and, during pregnancy, that the mother’s immune system will not attack the baby’s red blood cells. The main tests are ABO blood typing, Rh (D) typing, the antibody screen, and the crossmatch. Together, they identify a person’s blood group and check whether their blood is compatible with the blood they may receive. These tests are ordered before a planned transfusion or surgery, during pregnancy, and when a transfusion reaction is being investigated.

This article explains what each of these tests measures and what the results mean, so you can understand the terms on a transfusion or prenatal report.

Your ABO and Rh blood type is a stable characteristic that does not change over your lifetime. Even so, for safety, your blood is always retested and cross-checked before every transfusion, rather than relying on a previous result, a donor card, or memory. The antibody screen and crossmatch are repeated for each transfusion, because new antibodies can develop over time, especially after a previous transfusion or pregnancy. Your care team uses the results below together to make sure any blood you receive is compatible.

What are blood typing and compatibility tests?

Blood typing and compatibility tests identify a person’s blood group and check whether donor blood can be given safely. Blood groups are determined by markers, called antigens, on the surface of red blood cells. The immune system can make antibodies against the blood group antigens a person does not have. If someone receives blood of an incompatible type, these antibodies attack the transfused red blood cells and destroy them, a dangerous event called a hemolytic transfusion reaction. Blood typing and compatibility tests are done to prevent this by matching the blood group of the donor and the person receiving the blood, and by checking for antibodies that could cause a reaction.

How are the tests performed?

Blood typing and compatibility tests are performed on a sample of blood drawn from a vein, usually without any special preparation. Testing is done in a set order. First, the ABO and Rh type are determined. Next, an antibody screen looks for unexpected antibodies in the blood. Finally, if a transfusion is planned, a crossmatch checks the specific donor blood against the person’s own blood. When only the type and antibody screen are done ahead of a possible transfusion, this is often called a “type and screen.” When donor units are actually matched and set aside, it is called a “type and crossmatch.”

The individual tests

A transfusion or prenatal report may include some or all of the tests below.

ABO blood typing

ABO typing determines whether a person’s red blood cells carry the A antigen, the B antigen, both, or neither, which gives the four main blood types: A, B, AB, and O. People naturally have antibodies against the ABO antigens they lack, so matching the ABO type is the most important step in giving blood safely. Type O red blood cells lack both antigens and can be given to almost anyone in an emergency, which is why type O is called the universal red cell donor, while type AB can receive red blood cells of any ABO type. The laboratory confirms the result two ways: by testing the red cells for antigens and testing the plasma for the expected antibodies, and the two must agree.

Rh (D) typing

Rh typing determines whether the red blood cells carry the Rh (D) antigen. A person who has it is Rh positive, and a person who does not is Rh negative. About 85 percent of people are Rh positive. This matters because an Rh-negative person who is exposed to Rh-positive blood, through a transfusion or during pregnancy, can begin to make anti-D antibodies, which can cause problems with future transfusions or pregnancies. For this reason, Rh-negative people are given Rh-negative blood whenever possible.

Antibody screen

The antibody screen looks for unexpected antibodies against red blood cells, beyond the usual ABO antibodies. These antibodies can develop after a previous transfusion or pregnancy, when the immune system is exposed to red cell antigens it does not have. A negative screen means no unexpected antibodies were found, which is the most common result. A positive screen means an antibody was detected, and the laboratory then performs additional testing to identify it. A positive screen does not mean blood cannot be given, but it does mean the laboratory must find donor units that lack the matching antigen, which can take longer.

Crossmatch

The crossmatch is the final compatibility check before a transfusion. In it, the laboratory combines the person’s plasma with red blood cells from the specific donor unit to confirm they do not react. A compatible crossmatch means the donor unit can be given safely. An incompatible crossmatch means that unit cannot be used, and the laboratory looks for other compatible units. When the antibody screen is negative, a faster form of crossmatch may be used, while a positive screen requires a more thorough crossmatch.

Direct antiglobulin test (DAT, or direct Coombs test)

The direct antiglobulin test, also called the direct Coombs test, checks whether antibodies are already attached to a person’s own red blood cells. It is not part of routine pre-transfusion testing, but it is used to investigate specific problems, including a transfusion reaction, hemolytic anemia (in which red blood cells are destroyed by the immune system), and hemolytic disease of the newborn. A positive result means antibodies are coating the red blood cells and may be causing them to break down.

Blood typing and compatibility in pregnancy

Blood typing and compatibility tests have a special role in pregnancy, where the concern is the mother’s immune system reacting to the baby’s red blood cells. Early in pregnancy, the ABO and Rh type and an antibody screen are checked. The main issue is Rh incompatibility: when an Rh-negative mother carries an Rh-positive baby, the mother’s immune system can make anti-D antibodies that cross the placenta and destroy the baby’s red blood cells, causing a condition called hemolytic disease of the fetus and newborn, which can lead to anemia in the baby.

To prevent this, Rh-negative pregnant people are given a medication called Rh immune globulin (also known by the brand name RhoGAM), typically around 28 weeks of pregnancy and again within 72 hours after delivery if the baby is Rh positive, as well as after any event that could mix fetal and maternal blood. A newer option, increasingly used, is a blood test that detects the baby’s DNA in the mother’s blood to determine the baby’s Rh type before birth, which can help decide whether Rh immune globulin is needed. If the antibody screen is already positive, the pregnancy is monitored more closely, including antibody levels and ultrasound assessments of the baby.

What happens if a test is positive or incompatible?

An abnormal blood typing or compatibility result guides the next steps rather than preventing care. A positive antibody screen leads to further testing to identify the antibody, and to a search for donor blood that lacks the matching antigen, which can take extra time, so it is helpful for people with known red cell antibodies to carry a card listing them. An incompatible crossmatch simply means a particular donor unit cannot be used, and the laboratory finds compatible units instead. If a reaction to a transfusion is suspected, the transfusion is stopped, and tests such as the direct antiglobulin test are done to investigate. In pregnancy, a positive antibody screen leads to closer monitoring of the baby. In each case, the care team uses the results together to make sure any blood given is as safe as possible.

Questions to ask your doctor

What is my ABO and Rh blood type?
Was my antibody screen negative, or was an antibody found?
If an antibody was found, what is it, and how does it affect finding compatible blood?
Should I carry a card that lists my blood type or any red cell antibodies?
If I am pregnant and Rh negative, do I need Rh immune globulin, and when?
Could a test of the baby’s Rh type help decide whether I need Rh immune globulin?
If I had a reaction to a transfusion, what caused it, and what does it mean for future transfusions?
Do my results affect any future surgery, transfusion, or pregnancy?
How far in advance should compatibility testing be done before a planned procedure?
Who should I tell about my blood type and antibody results?