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    What is a Direct Antiglobulin Test (DAT), and What Does It Mean?

    Have you been told you need a Direct Antiglobulin Test (DAT) or seen it on your lab results and aren't sure what it means? This important blood test checks whether your immune system is attacking your red blood cells. When this occurs the red blood cells may be destroyed resulting in hemolysis (bursting of cells). In this article, we’ll break down what the DAT is, why your provider may have ordered it, how the test is done, and what your results might mean, so you feel informed and prepared every step of the way.

    Michele Palermo, MLS(ASCP), CLT(NYSED)

    Direct Antiglobulin Test (DAT) Information

    What Is a Direct Antiglobulin Test?

    Also known as a DAT, this test looks for antibodies attached to red cell antigens (proteins on the surface of your red blood cells) that coat your red blood cells. Red blood cells carry oxygen throughout the body. Because this test looks directly at the red blood cells in your sample, it's called a direct test. This test is also called a Direct Coombs test.

    When Would This Be Ordered?

    A DAT may be ordered after a complete blood count (CBC) test result shows an abnormal red cell distribution width (RDW), even if the other results look normal. The RDW tells us how much the size of your red cells varies. A high RDW can mean your bone marrow (the part of the bone that makes red blood cells) is producing extra blood cells, especially reticulocytes (immature red blood cells), to help replace the mature ones being destroyed. In this case, the question becomes: What destroyed the red blood cells? A DAT is part of answering this question.

    Why Is This Test Ordered?

    Your provider or the lab may order a DAT for many reasons. Common ones include:

    Newborn testing: To check for neonatal jaundice or ABO/Rh incompatibility between a mother and baby. This is most common with Type O or Rh-negative moms and may lead to a serious condition called Hemolytic Disease of the Fetus and Newborn (HDFN).
    Positive antibody screen: If a routine blood screen shows unexpected antibodies, the DAT helps confirm if your immune system is reacting and how strong the reaction is.
    Suspected transfusion reaction: If symptoms suggest your body is rejecting a recent blood transfusion, a DAT can help confirm that.
    Autoimmune testing: The test may help detect autoantibodies (which attack your red cells), like those seen in pernicious anemia or autoimmune hemolytic anemia (AIHA).
    Drug-related reactions: Some medications, like insulin, can rarely trigger antibodies that stick to red blood cells.
    Passive antibodies: Antibodies passed from donor plasma or immunoglobulin (rare with today's screening methods).
    Nonspecific protein interference: Sometimes, stray proteins can stick to red blood cells and confuse test results.
    Complement activation: From infection, autoimmune reactions, or transfused antibodies.
    Organ transplant reactions: In rare cases, antibodies from the donor's white cells (passenger lymphocytes) can attack your blood.
    Hemolysis workup: To find out if an autoimmune disease is causing red blood cell destruction.

    How Should You Prepare for the Test?

    This test requires no special preparations by the patient. A member of the clinical team will draw a pink-topped tube, and the blood bank will take it from there.

    What happens in the Lab?

    In the lab, a scientist will wash your red blood cells, use reagents (special chemicals), and look under the microscope for agglutination (clumping). If there's no agglutination, the test is considered negative. If agglutination is seen, it will be graded by size and reported.

    What Do the Results Mean?

    A positive DAT means something - usually IgG antibodies, complement, or both - is coating your red blood cells. What is positive tells us more:

    If all 3 reagents are negative, the test is negative.

    If the complement is positive, then results like autoimmunity, infection, or intrinsic red cell antigens may be the cause.

    If IgG is positive, then things like transfusion reactions, autoimmune hemolytic anemia, or ABO/RH are more likely to be the culprit.

    No matter what is positive, a DAT shows that your immune system is active, possibly in a way that's affecting your red cells and causing symptoms.

    Questions to Ask Your Provider

    What was my positive DAT due to?
    What does a DAT mean?
    Is a Direct Coombs test the same thing as a DAT?
    Is my hemoglobin a result of my positive DAT or another cause?

    Final Takeaway

    The DAT is a test that checks whether antibodies or immune proteins are attacking your red blood cells. It's often used in newborn testing, transfusion reactions, autoimmune workups, and more. It's simple and requires no prep, but still gives your provider helpful insight into what is happening inside your body.

    Sources:

    AABB. Technical Manual, 20th edition. Bethesda, MD: American Association of Blood Banks; 2020.

    Lab Tests Online. "Direct Antiglobulin Test (Coombs Test)." American Association for Clinical Chemistry. https://labtestsonline.org/tests/direct-antiglobulin-test

    CLSI. Standards for Blood Banking and Transfusion Services (BB01). Clinical and Laboratory Standards Institute; current edition.

    Flegel WA, Westhoff CM. The Direct Antiglobulin Test. In: Cappellini MD, Motta I, eds. Blood Transfusion. Bethesda (MD): National Center for Biotechnology Information (US); 2003–. Available from: https://www.ncbi.nlm.nih.gov/books/NBK547707/

    This information is not a substitute for, nor does it replace professional medical advice, diagnosis, or treatment. If you have any concerns or questions about your health, you should always consult with a healthcare professional.