It is never normal for circulating red cells to be coated with antibody. The direct antiglobulin test (DAT, direct Coombs') is used to detect abnormal in vivo coating of red cells with antibody globulin (IgG) or complement, or both.

When this test is performed, the red cells are taken directly from the sample, washed with saline (to remove residual globulins left in the client's serum surrounding the red cells but not actually attached to them), and mixed with antihuman globulin (AHG). If the AHG causes agglutination of the client's red cells, specific antiglobulins can be used to determine if the red cells are coated with IgG, complement, or both.

The most common cause of a positive DAT is autoimmune hemolytic anemia, in which affected individuals have antibodies against their own red cells. Other causes of positive results include hemolytic disease of the newborn, transfusion of incompatible blood, and red cell-sensitizing reactions caused by drugs. In the latter, the red cells may be coated with the drug or with immune complexes composed of drugs and antibodies that activate the complement system.^17,18 Drugs associated with such reactions are listed in Table 4-2. Positive DAT results may also be seen in individuals with Mycoplasma pneumonia, leukemias, lymphomas, infectious mononucleosis, lupus erythematosus and other immune disorders of connective tissue, and metastatic carcinoma. Other conditions, such as the aftermath of cardiac vascular surgery, are associated with production of autoantibodies.

• Negative (no agglutination)

• Many drugs may cause positive reactions (see Table 4-2).

• Suspected hemolytic anemia or hemolytic disease of the newborn as indicated by a positive reaction
• Suspected transfusion reaction as indicated by a positive result
• Suspected drug sensitivity reaction as indicated by a positive result

Nursing Care Before the Procedure

For samples collected by venipuncture, client preparation is the same as that for any study involving the collection of a peripheral or cord blood sample (see Appendix I).

• Drugs currently taken by the client should be noted.
• If the test is to be performed on the newborn, the parent(s) should be informed that a sample of umbilical cord blood will be obtained at delivery and will not result in blood loss to the infant.

A venipuncture is performed and the sample collected in a red-topped tube or other type of blood collection tube, depending on laboratory preference. For cord blood, the sample is collected in a red- or lavender-topped tube (depending on the laboratory) from the maternal segment of the cord after it has been cut and before the placenta has been delivered.

Nursing Care After the Procedure

For venipunctures, care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Complications and precautions: Note and report a positive value in cord blood of a neonate with possible erythroblastosis fetalis for direct Coombs' because this test result indicates that antibodies are attached to the circulating erythrocytes. Assess associated bilirubin and hemoglobin levels. Prepare the infant for exchange transfusion of fresh whole blood that has been typed and cross-matched with the mother's serum.