Determination of Blood Group by Tube Method
Discover the reliability of the test tube method for accurate blood group determination. Learn how this method enhances antigen-antibody interactions, minimizes false positives, and outperforms traditional slide tests in blood typing.
Highlights
- The test tube method is the gold standard for blood group determination, offering superior reliability by enhancing antigen-antibody interactions through centrifugation, making it essential for accurate blood typing.
- Learn why the test tube method outperforms the slide method in blood group testing, providing detailed insights into its accuracy and reliability in detecting even weak antigen-antibody reactions.
- Ensure precise blood group identification with the test tube method, which minimizes false positives and optimizes reagent use, setting the benchmark for dependable blood typing protocols.
The test tube method of blood grouping is regarded as more reliable than the slide method, despite requiring more time and equipment. In cell grouping, the patient’s red cells, which have been washed in saline, are mixed with a known antiserum in a test tube. This mixture is incubated at room temperature and then centrifuged. For serum grouping, the patient’s serum is combined with reagent red cells of a known group, which can be commercially obtained or prepared in the laboratory. After incubation and centrifugation, a sediment of red cells, known as a cell button, forms at the bottom of the tube. This sediment is gently dislodged by tapping the tube and is then examined for signs of agglutination.
Positive (+) Test
A positive test is indicated by clumps of red cells suspended in a clear fluid. Agglutination in the tube test is graded from 1+ to 4+ and is read macroscopically, as depicted in figure 1. The grading is as follows:
- Negative: Uniform suspension of red cells with no agglutination.
- Grade 1+ (1+): Many small clumps of red cells, giving a fine granular appearance.
- Grade 2+ (2+): Numerous large clumps with a significant number of free red cells.
- Grade 3+ (3+): Three or four distinct clumps with a few free red cells.
- Grade 4+ (4+): A single solid clump of red cells with no free red cells.
Negative (–) Test
A negative test result is characterized by a uniform suspension of red cells without agglutination.
It is essential to set up separate tubes for auto-control, positive control, and negative control alongside the test sample tube. The auto-control tube contains a mixture of the patient’s red cells and the patient’s own serum. This control is crucial for ruling out false-positive results caused by autoantibodies in the patient’s serum, which might lead to the autoagglutination of the patient’s own red cells. The auto-control test is particularly important when ABO grouping is performed using only the forward method and the blood group is typed as AB. The presence of autoantibodies in the recipient’s serum can result in positive outcomes for ABO grouping, Rh typing, antibody screening, and crossmatching.
In the two positive control tubes, anti-A serum is mixed with group A red cells, and anti-B serum is mixed with group B red cells. Conversely, in the two negative control tubes, anti-A serum is combined with group B red cells, and anti-B serum is mixed with group A red cells. These controls are necessary to verify that the reagents are functioning correctly.
| Forward (Cell) Grouping | Reverse (Serum) Grouping | Interpretation | ||
|---|---|---|---|---|
| Anti-A Serum | Anti-B Serum | A₁ Cells | B Cells | Blood Group |
| + | − | − | + | A |
| − | + | + | − | B |
| − | − | + | + | O |
| + | + | − | − | AB |
| Positive (+): Agglutination of red cells; Negative (−): No agglutination of red cells. | ||||
Why test tube method of blood grouping is more reliable than slide method?
The test tube method is preferred over the slide method due to its enhanced reliability. Centrifugation in the test tube method improves the reaction by bringing antigens and antibodies closer together, which allows for the detection of weaker antigen-antibody reactions. Moreover, this method avoids drying and requires smaller quantities of reagents.
If all tests—forward grouping, reverse grouping, and autocontrol—yield positive results, it may indicate the presence of a cold-reactive autoantibody. In such cases, it is advisable to wash the red cells with normal saline before performing forward typing to remove the antibody. Additionally, before conducting reverse grouping, the autoantibody should be adsorbed by washed cells until the autocontrol test returns a negative result.
Cite this page:
- Posted by Dayyal Dg.