Positive (+) Test
Clumps of red cells suspended in a clear fluid. Agglutination in tube test is graded from 1+ to 4+ and read macroscopically (See Figure).
Negative (–) Test
Uniform suspension of red cells.
Separate tubes of auto-control, positive control, and negative control should always be setup along with the test sample tube. Auto-control tube consists of mixture of patient’s red cells and patient’s own serum. This is required to rule out false-positive result due to auto antibodies in patient’s serum causing auto agglutination of patient’s own red cells. Auto-control test is particularly essential when ABO grouping is being done only by forward method and blood group is typed as AB. If there are auto antibodies in recipient’s serum, ABO grouping, Rh typing, antibody screening, and cross matching all will show positive result.
In two positive control tubes, anti-A serum is mixed with group. A red cells and anti-B is mixed with group B red cells respectively. In two negative control tubes, anti-A serum is mixed with group B red cells and anti-B serum is mixed with group. A red cells respectively. These controls are necessary to confirm that reagents are working properly.
Why test tube method of blood grouping is more reliable than slide method?
Test tube method of blood grouping is more reliable than slide method. This is because centrifugation enhances the reaction by bringing antigen and antibodies closer together and allows detection of weaker antigen antibody reactions; in addition drying is avoided and smaller amounts of reagent are required.
If forward grouping, reverse grouping, and autocontrol tests are all positive, then these results are probably indicative of a cold-reactive autoantibody. Before performing forward typing, red cells should be washed with normal saline to elute the antibody. Before performing reverse grouping, autoantibody should be adsorbed by washed cells till autocontrol is negative.