How to Determine Positive and Negative Blood Types

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Experts say that knowing your blood type is important, especially if you have frequent blood transfusions or you are trying to become pregnant. [1] The ABO blood system labels blood types with the letters A, B, AB, and O. Your blood also has a Rhesus or Rh factor, which can be either negative or positive. You inherit your blood type and Rh factor from your parents. Studies show that to determine your Rh factor, you can find out your parents’ Rh factors for information or take a blood typing test at your physician’s office. [2]

Part 1
Part 1 of 2:

Using Existing Information to Determine Rh Factor

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    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. [3]
    • 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. [4]
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    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 [5] (except in certain life-threatening emergencies when you might need to receive Rh+ blood).
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    Ask your parents what their Rh factors are. It can be possible to learn your Rh factor by analyzing your parents’ factors. [6] 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-. [7]
    • 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. [8]
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Part 2
Part 2 of 2:

Getting a Blood Type Test

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    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.
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    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. [9] 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. [10]
    • If you feel like you might pass out, tell the medical professional. He/She can help you recline. [11]
    • 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. [12]
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    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-. [13]
    • It is likely that the lab will run tests to determine your ABO blood type as well during this time.
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    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.
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    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. [14]
    • 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. [15]
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Expert Q&A

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  • Question
    If one parent has a positive blood type and the other has a negative blood type, what are the possible blood groups and rhd combinations of their offspring?
    Mandolin S. Ziadie, MD
    Board Certified Pathologist
    Dr. Ziadie is a board certified Pathologist in South Florida. She specializes in renal, transplant, and pediatric Pathology and has over 12 years of experience. She earned her medical degree from the University of Miami School of Medicine in 2004 and completed her fellowship in Pediatric Pathology at Children’s Medical Center in 2010.
    Mandolin S. Ziadie, MD
    Board Certified Pathologist
    Expert Answer
    Depending on the genetic makeup of the individuals, a positive/negative parental pairing can yield both a positive and negative blood type offspring OR only a positive blood type offspring. This is because each parent has two copies of the Rh gene. A negative blood type parent must have two negative copies, which causes them to lack the Rh factor. A positive blood type parent can have two positive copies OR one positive copy and one negative copy.
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  • Question
    Why are there positive and negative blood types?
    Mandolin S. Ziadie, MD
    Board Certified Pathologist
    Dr. Ziadie is a board certified Pathologist in South Florida. She specializes in renal, transplant, and pediatric Pathology and has over 12 years of experience. She earned her medical degree from the University of Miami School of Medicine in 2004 and completed her fellowship in Pediatric Pathology at Children’s Medical Center in 2010.
    Mandolin S. Ziadie, MD
    Board Certified Pathologist
    Expert Answer
    Positive and negative blood types refer to the presence or absence of a marker called the D-antigen (or Rh factor). This marker is located on the surface of your red blood cells. Blood from people who have this antigen is called "positive." Blood from people who do not have this antigen is called "negative."
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    If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow
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      References

      1. ↑ https://www.redcrossblood.org/donate-blood/blood-types.html
      2. ↑ https://www.mayoclinic.org/tests-procedures/rh-factor/about/pac-20394960
      3. ↑ http://www.mayoclinic.org/tests-procedures/rh-factor/basics/definition/prc-20013476
      4. ↑ http://www.mayoclinic.org/tests-procedures/rh-factor/basics/definition/prc-20013476
      5. ↑ https://www.nlm.nih.gov/medlineplus/ency/article/003345.htm
      6. ↑ http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/rh_factor.html
      7. ↑ http://www.mayoclinic.org/tests-procedures/rh-factor/basics/why-its-done/prc-20013476
      8. ↑ http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/rh_factor.html
      9. ↑ http://www.nhs.uk/conditions/Blood-tests/Pages/Introduction.aspx
      1. ↑ http://www.nhs.uk/conditions/Blood-tests/Pages/Introduction.aspx
      2. ↑ http://www.nhs.uk/conditions/Blood-tests/Pages/Introduction.aspx
      3. ↑ https://www.nlm.nih.gov/medlineplus/ency/article/003345.htm
      4. ↑ https://www.nlm.nih.gov/medlineplus/ency/article/003345.htm
      5. ↑ https://www.acog.org/womens-health/faqs/the-rh-factor-how-it-can-affect-your-pregnancy
      6. ↑ https://www.acog.org/womens-health/faqs/the-rh-factor-how-it-can-affect-your-pregnancy

      About This Article

      Article Summary X

      To determine if you have a positive or negative blood type, check your medical records to see if your blood's Rh factor has already been tested. Alternatively, ask your parents what their blood type is, since if they're both negative, you will be too. If not, ask your doctor for a blood type test. For the test, let a nurse locate a vein on the inside of your arm and take blood with a needle. Then, wait to receive your blood type result from a lab. For tips on how your blood type can affect your pregnancy, read on!

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