Blood Type and Rh Factor
Also written as: ABO and Rh typing, Blood group
Which ABO group (A, B, AB, or O) and Rh factor (positive or negative) your red cells carry.
What it measures
Which inherited markers, called antigens, sit on the surface of the red blood cells - the A and B antigens that define the ABO group, and the RhD antigen that defines positive or negative status. These markers determine which donated blood the immune system will tolerate.
Why a doctor orders it
It is ordered before planned surgery and transfusions, at the start of prenatal care, and as part of blood donation. In pregnancy, Rh status matters because a difference between an Rh-negative parent and an Rh-positive fetus is something prenatal care routinely screens for and addresses.
What is generally associated with a higher result
This test reports a category, not a level. The possible results pair an ABO group (A, B, AB, or O) with an Rh status (positive or negative), giving eight common types. O negative red cells can generally be given to recipients of any type in an emergency, and people who are AB positive can generally receive red cells of any type - the origin of the terms universal donor and universal recipient.
What is generally associated with a lower result
No blood type is healthier than another. Rh-negative status carries practical relevance mainly in pregnancy and transfusion planning, both of which have standard, well-established protocols built around it.
Everyday things that move this number
Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:
- A recent transfusion, which can temporarily complicate typing
- A prior bone marrow or stem cell transplant, which can genuinely change a person's blood type
- Pregnancy, which can affect the antibody screen usually run alongside the typing
- Certain medications and conditions that interfere with the antibody screen rather than the type itself
Questions to take to your appointment
- Is my Rh status relevant to a current or planned pregnancy?
- Was an antibody screen run with the typing, and did it find anything?
- Does the hospital keep my type on file, and should I carry a record of it?
- Is it true the hospital will re-check my type before any transfusion regardless of what I carry?
Build a printable questionnaire with your own results →
Where to read more
MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .
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