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The Direct Coomb's Test

written by: Teresa Martin•edited by: Emma Lloyd•updated: 7/1/2010

Direct Coomb's testing identifies the presence of antibody coating the cells. Save transfusion practices require identification of what is coating the cells and why.

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    Direct Coomb's Testing

    A direct Coombs test is commonly used to determine if antibodies have bound to red blood cells. On the cells are antigenic sites and these are the points of connection with antibodies. The antibodies can be present following a transfusion where some of the transfused cells are recognized as foreign. The immunologic response by the body causes a production of antibodies. It may take a few days for the titer of the antibodies to reach a detectable level in the blood. Prior to this, antibodies can bind to the red blood cells, effectively coating them. Direct Coombs testing following a suspected transfusion aids in identifying the specific antibody that may have been involved in the transfusion reaction.

    In addition to alloantibodies, the body sometimes produces autoantibodies. These are antibodies against the body itself. The body begins to fight against itself by forming antibodies. Conditions in which this occurs are lupus erythematosus and mixed connective tissue disease.

    Drugs can also coat cells so it is important for the physician to evaluate the medications a patient may be taking in determining the reason for the positive direct Coombs test.

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    How the Test Is Done

    Testing involves drawing a blood sample and separating the cells from the serum. The cells are washed with saline to clear any of the patient serum from the cells. This is followed by adding anti-human globulin to the washed patient cells and a period of centrifugation. If antibody has been bound to antigenic sites on the cell, clumping will occur. The antibodies will either be IgM or IgG and the anti-human globulin binds with them forming a lattice. The larger amount of antibody present on the cells corresponds to larger clumps.

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    Direct Coombs Testing of Newborns

    Direct Coombs testing is done on newborn babies when there is a possibility of incompatibility with the mother's blood which results in hemolytic disease of the newborn. This most commonly occurs when the mother has Rh negative blood and the baby has Rh positive blood. The mother's antibodies cross the placental barrier and enter the fetus' blood. These antibodies will bind to the baby's cells resulting in hemolysis of the cells. Depending on the severity of the condition, which is directly related to the amount of antibodies that have been formed, fetal transfusion may be required to provide adequate cells for the transport of oxygen.

    ABO incompatibility can also result in hemolytic disease of the newborn. Usually the mother has type O blood and the baby will be type A, B or AB. In this situation, the mother's naturally occurring A and B antibodies can cross the placental barrier and enter the fetus bloodstream. These antibodies bind to the cells of the baby resulting in hemolysis and destruction of the baby's cells.

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