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Blood Type Test

Serious transfusion reactions are rare today because of blood type tests.

Rh test

Rh blood type checks for the Rh antigen (also called the Rh factor) on red blood cells. If your red blood cells:

  • Have the Rh antigen, your blood is Rh-positive.
  • Do not have the Rh antigen, your blood is Rh-negative.

For example, if you have the A and Rh antigens, your blood type is A-positive (A+). If your blood has the B antigen but not the Rh antigen, your blood type is B-negative (B–).

Rh blood type is especially important for pregnant women. A problem can occur when a woman who has Rh-negative blood becomes pregnant with a baby (fetus) that has Rh-positive blood. This is called Rh incompatibility. If the blood of an Rh-positive baby mixes with the blood of an Rh-negative mother during pregnancy or delivery, the mother's immune system makes antibodies. This antibody response is called Rh sensitization and, depending on when it occurs, can destroy the baby's red blood cells.

Rh sensitization does not generally affect the health of the baby during the pregnancy in which the sensitization occurs. But the health of a baby with Rh-positive blood during a future pregnancy is more likely to be affected. After sensitization has occurred, the baby can develop mild to severe problems (called Rh disease or erythroblastosis fetalis). In rare cases, if Rh disease is not treated, the baby may die.

An Rh test is done in early pregnancy to check a woman's blood type. If she is Rh-negative, she can get a shot of Rh immunoglobulin that almost always prevents sensitization from occurring. Problems from Rh sensitization have become very rare since Rh immunoglobulin was developed.

Why It Is Done

A blood type test is done:

  • Before a person gets a blood transfusion.
  • Before a person donates blood.
  • Before a person donates an organ for transplantation.
  • Before surgery.
  • When a woman is planning to become pregnant or first becomes pregnant.
  • To show whether two people could be blood relatives.
  • To check the identify of a person suspected of committing a crime.

How To Prepare

You do not need to do anything before you have this test.

How It Is Done

The health professional drawing blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. If the needle is not placed correctly or if the vein collapses, more than one needle stick may be needed.
  • Hook a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Put a gauze pad or cotton ball over the needle site as the needle is removed.
  • Put pressure to the site and then a bandage.

WebMD Medical Reference from Healthwise

Last Updated: March 12, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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