Blood Type (ABO/Rh)

What is a blood type?

A blood type is a classification based on inherited antigens found on the surface of red blood cells. In the ABO system, the cell surface may carry the A antigen, the B antigen, both or neither; accordingly, the blood type is called A, B, AB or O. In the Rh system, the presence of the D antigen is described as Rh positive and its absence as Rh negative.

In the laboratory, the blood sample is mixed with reagents containing anti-A, anti-B and anti-D antibodies, and the cells are checked for clumping. To confirm the ABO group, the natural antibodies in the plasma are also tested against known A and B cells; this is called reverse grouping. Agreement between the two methods increases the reliability of the result.

When is a blood type test ordered?

Blood type is determined especially during pregnancy follow-up, before planned surgery and at blood donation. When an Rh-negative mother carries an Rh-positive baby, the mother's immune system may develop antibodies against the baby's blood cells, so it is important to know the Rh status early in pregnancy. Many people also have this test to learn their own blood type and keep it on record for emergencies.

• Determining the expectant mother's blood type and Rh status at the start of pregnancy
• Preparation before planned surgery or an interventional procedure
• Initial assessment before a transfusion of blood or blood products
• Blood, organ or stem cell donation processes
• Learning your blood type for your personal health records
• Assessment of the newborn when blood incompatibility between mother and baby is suspected

How should you prepare for a blood type test?

For a blood type test, you do not need to fast, wait for a particular time of day or pause your medicines. If you have recently had a blood transfusion or have had a bone marrow transplant in the past, be sure to share this, as the result may be misleading. If your blood type has previously been determined elsewhere, you can also mention it for comparison.

A small amount of blood is drawn from a vein in the arm into a purple-top tube. The blood draw can also be done by a nurse at your home or workplace; the whole process usually takes no more than a few minutes.

What does the blood type result mean?

This test does not include a numerical value or a reference range; the result is reported as group A, B, AB or O and as Rh positive or Rh negative. Your blood type does not show whether you are healthy or ill; it only describes an inherited trait and, except in cases such as a bone marrow transplant, does not change throughout life. In some people found to be Rh negative, an additional test may be done to look for a weak D antigen.

If you are an Rh-negative expectant mother, your doctor may plan antibody screening during pregnancy and, if needed, preventive measures. When a transfusion is needed, the hospital blood bank determines the blood type again for safety and performs a crossmatch test; therefore, a previously obtained result does not replace these procedures. Because plasma antibodies have not yet developed in infants, the result may be confirmed later on. A doctor should evaluate what your result means for you, together with your health status and other tests.