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Rh Blood Typing

Rh Blood Typing



Rh blood typing is performed in order to determine the Rh factor of an individual’s blood. The term “Rh factor” refers to an antigen on the surface of the red blood cells. All red blood cells have certain substances on their surfaces. These substances are called “antigens,” and may be molecules of protein, carbohydrate, glycolipid, or glycoprotein. Blood typing categorizes blood by identifying the presence or absence of these antigens on the surface of the red blood cell.

The Rh system identifies the presence (denoted as positive) or absence (denoted as negative) of a particular antigen termed the Rhesus antigen. Its names stems from the fact that the Rh factor was first identified on the red blood cell surfaces of Rhesus monkeys. When the Rh factor is present on the surface of the red blood cell, the blood is said to be Rh-positive; when the Rh factor is absent from the surface of the red blood cell, the blood is said to be Rh-negative.

The Rh factor status is reported in conjunction with identification of the major ABO blood group of the individual. The ABO blood group system identifies a type of protein antigen on the red blood cell surface as Type A, Type B, Type AB, or Type O. An individual’s blood type, then is reported as a combination of information obtained about the ABO and RH blood group systems; for example, A-positive, or A-negative, etc.

Blood typing is particularly important when an individual needs to receive a blood transfusion. If the wrong blood type is given, there is a high risk of an adverse transfusion reaction. For example, the first time an Rh-negative individual is given blood from an Rh-positive donor, there will probably not be any problem. However, if the Rh-negative individual receives future transfusions of Rh-positive blood, the recipient’s immune system will recognize the Rh antigen on the donor blood as foreign, and will begin to produce antibodies directed against that antigen. The antibodies will attack the donor blood, damaging and bursting the donor red blood cells. This results in high serum levels of hemoglobin spilling from the burst red blood cells (called hemoglobinemia), disseminated intravascular coagulation or DIC (a condition in which clotting factors are used up very rapidly, resulting in the potential for severe, uncontrollable bleeding), kidney failure, and eventually complete cardiovascular collapse (a combination of heart attack, shock, and lack of blood flow to all major organs and tissues).

Knowing a pregnant woman’s Rh-factor is crucial because there is always a chance during pregnancy, labor, and delivery, that some of the baby’s blood will get into the mother’s bloodstream. If this happens in an Rh-negative mother with an Rh-positive baby, the mother’s body will identify the baby’s Rh-negative blood as foreign and begin producing antibodies against the Rh-factor. This is called Rh-sensitization. The first time this sensitization occurs between a mother and her baby, the baby usually doesn’t suffer any ill-effects. But in subsequent pregnancies, if the mother is again carrying an Rh-positive baby, having already been exposed to the Rh-antigen previously, her body will begin to produce Rh-antibodies more quickly and in greater numbers. If these cross over into the baby’s bloodstream, they can begin destroying the baby’s red blood cells, resulting in severe illness. This problem is referred to as Rh disease, hemolytic disease of the newborn, or erythroblastosis fetalis. In order to avoid this problem, Rh testing is done prior to pregnancy or early in pregnancy. Rh-negative women can be given a special shot called Rh-immune globulin which can prevent Rh-sensitization.


Blood typing is ordered prior to a blood transfusion, to make sure that the donor blood type is appropriately compatible with the recipient’s blood type. It is also done on donor blood, on a donor who is giving an organ to be used for transplantation, as well as prior to surgery (so that the patient’s blood type is known, should the individual needs an unexpected, emergency blood transfusion). Rh-typing is also important in pregnant women. When the mother and the baby have different Rh-types, there is a risk to the baby of illness caused by the mother’s antibodies; if the mother is identified as having Rh-negative blood, a shot called Rh-immune globulin can prevent the problem from developing.


Some situations may confuse the results of blood typing, including recent x-ray test using contrast, use of medications such as methyldopa, levodopa, and certain antibiotics (including cephalexin). Other factors that may confuse test results include having received a blood transfusion in the previous three months, having had a bone marrow transplant in the past, or having a history of cancer or leukemia.


This test requires blood to be drawn from a vein (usually one in the forearm), generally by a nurse or phlebotomist (an individual who has been trained to draw blood). A tourniquet is applied to the arm above the area where the needle stick will be performed. The site of the needle stick is cleaned with antiseptic, and the needle is inserted. The blood is collected in vacuum tubes. After collection, the needle is withdrawn, and pressure is kept on the blood draw site to stop any bleeding and decrease bruising. A bandage is then applied.


There are no restrictions on diet or physical activity, either before or after the blood test.


Disseminated intravascular dissemination— A condition in which the clotting factors in the blood are rapidly used up, resulting in a severe deficit in clotting factors and a very high risk of severe, uncontrollable bleeding.

Erythroblastosis fetalis— A condition in which the incompatability between a mother’s Rh-negative blood type and a baby’s Rh-positive blood type results in destruction of the baby’s red blood cells by maternal antibodies.


As with any blood tests, discomfort, bruising, and/or a very small amount of bleeding is common at the puncture site. Immediately after the needle is withdrawn, it is helpful to put pressure on the puncture site until the bleeding has stopped. This decreases the chance of significant bruising. Warm packs may relieve minor discomfort. Some individuals may feel briefly woozy after a blood test, and they should be encouraged to lie down and rest until they feel better.


Basic blood tests, such as Rh blood typing, do not carry any significant risks, other than slight bruising and the chance of brief dizziness.


Rh blood typing reports back whether the individual’s red blood cells have the Rh antigen present on their surface (Rh-postive) or absent from their surface (Rh-negative). About 84% of all people are Rh-positive; about 16% are Rh-negative.



Goldman L, Ausiello D., eds. Cecil Textbook of Internal Medicine. 23rd ed. Philadelphia: Saunders, 2008.

Hoffman R. et al. Hematology: Basic Principles and Practice. 4th ed. Philadelphia: Elsevier, 2005.

McPherson RA et al. Henry’s Clinical Diagnosis and Management By Laboratory Methods. 21st ed. Philadelphia: Saunders, 2007.


American Association of Clinical Chemistry. 1850 K St., N.W Suite 625, Washington, DC 20006.


National Institutes of Health. [cited February 10, 2008].

Rosalyn Carson-DeWitt, MD

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