Blood group

Blood grouping shows your AB0 group (A, B, AB or 0) and whether you are Rh positive or Rh negative. An indirect antiglobulin test (IAT) is performed alongside grouping to detect antibodies against other people’s red blood cells. These details matter in pregnancy, before surgery and procedures, and when you want to know your blood group.

Who is this test for

• pregnant women at the beginning of pregnancy as part of basic testing, together with IAT,
• people before surgery or a procedure potentially requiring transfusion, when requested by a doctor or hospital,
• people needing an urgent result, for example before a scheduled procedure, because urgent blood grouping is also available,
• anyone wanting to know their blood group, for example for personal medical records.

What blood group is and what testing shows

Blood group depends on antigens, special molecules on red blood cell surfaces. In the AB0 system, a person with group A has antigen A, group B antigen B, group AB both antigens and group 0 neither. Blood group is inherited from parents.

From early childhood, blood contains antibodies against AB0 antigens that the person lacks: group A has anti-B, group B anti-A, group 0 both, and group AB neither. These antibodies mean incompatible transfusion destroys the transfused red blood cells and may cause a severe transfusion reaction.

The Rh factor is determined by antigen D: people with it are Rh positive; people without it are Rh negative. Unlike the AB0 system, antibodies to D do not occur naturally. They form only when an Rh-negative person encounters Rh-positive blood cells, after transfusion or during pregnancy with an Rh-positive baby.

IAT (indirect Coombs test) looks for such acquired, so-called irregular antibodies in blood. Besides Rh, other blood group systems exist, such as Kell, Duffy and Kidd; antibodies to their antigens can also matter in pregnancy and transfusion.

Before transfusion itself, the hospital always checks blood group and compatibility using its own samples, so a blood group report does not replace pre-transfusion testing.

Blood group and IAT in pregnancy

If the mother is Rh negative and the baby inherits Rh-positive blood from the father, her body may produce antibodies against the baby’s red blood cells. This usually happens during birth; in a subsequent pregnancy, these antibodies may cross the placenta and cause haemolytic disease of the fetus and newborn. Antibody formation is prevented with anti-D immunoglobulin given to an Rh-negative woman as decided by a doctor, for example after birth of an Rh-positive baby, miscarriage or bleeding in pregnancy.

IAT also detects antibodies against other antigens such as Kell, so it is performed in Rh-positive pregnant women too. In Rh-negative pregnant women, and whenever antibodies are found, the gynaecologist determines follow-up tests during pregnancy. Read more in the article on indirect antiglobulin test monitoring in pregnancy.

Preparation and sample

Blood grouping requires venous blood. Fasting and special preparation are not required. At blood collection, mention pregnancy, transfusion within the past several months or anti-D immunoglobulin administration, because these affect IAT interpretation.

How results are interpreted

The report states the AB0 blood group and Rh factor, for example “A Rh positive” or “0 Rh negative”, and the IAT result.

• Negative IAT means no irregular red blood cell antibodies were found in blood.
• Positive IAT requires further investigation to identify and quantify the antibody. It may also result from administered anti-D immunoglobulin. A doctor assesses the significance for pregnancy or future transfusion.

How to arrange testing

You can come for blood collection without an appointment or referral at one of our branches throughout Croatia (listed on the Contact). Prices for all tests and panels, including urgent blood grouping, are listed in the service price list.

Frequently asked questions

What is IAT and why is it performed in pregnancy?

IAT, the indirect antiglobulin test, shows whether your blood contains antibodies against other people’s red blood cells. It is performed in pregnancy because these antibodies can cross the placenta and damage the baby’s red cells. All pregnant women are tested at the start of pregnancy, with follow-up set by a gynaecologist.

Can parents with blood groups A and B have a child with group 0?

Yes. A child inherits one blood group gene from each parent, and the group 0 gene is not expressed alongside gene A or B. A group A parent may carry A and 0, and a group B parent B and 0, so a child can inherit two 0 genes and have group 0.

Can blood group change?

Generally not. An exception is bone marrow or blood-forming stem cell transplantation from a donor of another group, after which the recipient gradually acquires the donor’s group. Transfusion does not change blood group but may temporarily make determining it harder.

Is Rh-negative blood dangerous in pregnancy?

Not by itself. It is important that your gynaecologist knows you are Rh negative so that IAT is monitored and anti-D immunoglobulin given in time. This monitoring largely prevents antibody formation.