From the Guidelines
To confirm an immune reaction during a blood transfusion, the direct antiglobulin test (DAT), also known as the direct Coombs test, is the primary laboratory test recommended. This test detects antibodies or complement proteins bound to red blood cells, which indicates an immune reaction is occurring. When a transfusion reaction is suspected, blood samples should be drawn immediately from the patient and sent to the laboratory along with the transfusion bag for testing.
Key Laboratory Tests
- Direct antiglobulin test (DAT)
- Crossmatching of pre- and post-transfusion samples
- Antibody screening
- Elution studies to identify the specific antibodies involved
- Complete blood count
- Hemoglobin
- Haptoglobin
- Bilirubin
- Urinalysis for hemoglobinuria These tests help determine whether the reaction is due to ABO incompatibility, development of alloantibodies, or other immune-mediated processes, as suggested by the American Society of Clinical Oncology clinical practice guideline update 1. Prompt testing is crucial as it guides management decisions and helps prevent future transfusion reactions by identifying antibodies that need to be avoided in subsequent transfusions. Approximately 90% of patients who are alloimmunized will have alloantibody to HLA antigens, as noted in the study 1.
From the Research
Laboratory Tests for Immune Reactions
The following laboratory tests can be used to confirm an immune reaction during blood transfusion:
- Direct Antiglobulin Test (DAT): This test detects immunoglobulin and/or complement on the surface of red blood cells 2, 3.
- Cross-matching: This test is used to prevent hemolytic transfusion reactions in allo-immunized or new-born patients found positive with direct antiglobulin test 4.
Interpretation of Test Results
The results of these tests must be viewed in light of clinical and other laboratory data 2. False reactions may occur with improper technique, including improper washing, centrifugation, and specimen agitation at the time of result interpretation 2. Patient factors, such as spontaneous red blood cell agglutination, may also contribute to false results 2.
Limitations of the Tests
The DAT has limitations, including false-positive and false-negative results 3. In cases where the DAT is negative, but the phenotype is compatible with warm-antibody type autoimmune hemolytic anemia, alternative methods may be used to detect low levels of IgG sensitization 5. These methods include using 4°C, low ionic strength washes to prepare the cells for the DAT reaction, and testing for sensitization with an anti-IgA, and, if necessary, an anti-IgM reagent 5.