How to Determine Positive and Negative Blood Types

Using Existing Information to Determine Rh Factor

Understand what determines your Rh factor. Located on your red blood cells, the Rh factor is a protein you did or did not receive from your parents. You are Rh positive when you have the protein. If the protein is not present, you are Rh negative. People with positive Rh factors have positive blood types, whether A+, B+, AB+, or O+. People with negative Rh factors then have negative blood types, e.g. A-, B-, AB-, or O-. The majority of people have Rh positive factors.

Check your medical charts. It is possible that while having other bloodwork done, your Rh factor was tested as well. Ask your primary physician whether he/she has a blood type for you on record. If you have had blood transfusions regularly, your blood type is likely recorded. The same applies if you are a blood donor. If you have a positive Rh factor, you can receive either Rh+ or Rh- blood during transfusions. If you have Rh- blood, you must receive Rh-. blood (except in certain life-threatening emergencies when you might need to receive Rh+ blood).

Find out your parents' Rh factors. Ask your parents what their Rh factors are. It can be possible to learn your Rh factor by analyzing your parents’ factors. If your parents both have Rh- blood, you will most likely be Rh- (see bullet point for exceptions). If your mother is negative and your father is positive (or vice versa), you could be either positive or negative. In this type of situation, you will need more definite evidence from a lab test performed by your doctor or blood collection centers. It should be noted that if both of your parents are Rh+, you could still be Rh-. Because people with positive blood types can possess either two positive Rh genes (Rh+/Rh+) or one positive and one negative (Rh+/Rh-), there can be instances where both parents have positive blood types but their children have negative blood types.

Getting a Blood Type Test

Ask your doctor for a blood type test. If your parents have different Rh factors (or if they are both positive and you want to know for sure that you are also positive), you can request a blood typing test. This outpatient procedure is fast and should not be very painful. You can go home afterwards.

Take blood typing test. A nurse or physician will clean a spot on your inner elbow or wrist with an antiseptic towelette. He/She will locate an easily accessible vein in this area. After wrapping a tourniquet around your upper arm to pool the blood, he/she will stick your vein with a needle. The needle is usually connected to a syringe, which removes your blood. Once he/she has taken enough blood, he/she will take out the needle and gently press the injection site with a sterile swab. You will then receive a bandage. Afterwards, the nurse will label your sample and send it to a lab for testing. Doctors test children’s blood through the back of their hands. If you feel like you might pass out, tell the medical professional. He/She can help you recline. You might feel a prick, sting, or mild pain when your nurse sticks you. After the test, you might bruise at the puncture site. This pain should not last long.

Test blood sample. At the laboratory, a technician will test your sample for its Rh factor. He/She will combine a sample of your blood with an anti-Rh serum. If your cells clot, you have Rh+. On the other hand, if your blood cells do not stick together, you are Rh-. It is likely that the lab will run tests to determine your ABO blood type as well during this time.

Recognize the importance of your results. Record your blood type in a safe place and share this information with your emergency contacts. You will need this information if you require a blood transfusion or organ transplants. Additionally, if you are or plan to be pregnant, knowing your Rh factor is essential.

Be aware of your pregnancy risks. If you are female and Rh-, you will need to have your partner test his Rh factor. If you are Rh- and he is Rh+, you can experience Rh incompatibility. This means that if your baby inherits his father’s Rh+, your antibodies can attack the baby's red blood cells. This can cause extreme anemia and possible death for the baby. During your pregnancy, if you are Rh-, you will take blood tests to see whether your body is producing antibodies against Rh+ blood. The first will happen in your first trimester and the second will occur when you are 28 weeks pregnant. If no antibodies have developed, you will receive a shot of Rh immune globulin. This shot will prevent your body from producing harmful antibodies against your child. If your body has produced antibodies against Rh+ blood, you will not receive an immune globulin shot. Instead, doctors will keep a close eye on your baby as he/she develops. Either before or after his/her birth, your child might receive a blood transfusion. Once your baby is born, doctors can test his/her Rh factor. If your baby shares your Rh factor, you will need no further treatment. If you are Rh- negative, however, and your baby is Rh+, you must receive another immune globulin shot.