Requirements for Single tube compatibility technique using AHG reagent

The following are required:
1. Patient’s serum

2. Donor’s washed 3% red cell suspension
prepared as follows:

  • Transfer 0.2–0.5 ml of red cells from the pilot tube of the donor blood into about 5 ml of physiological saline  and mix. 
  • Centrifuge at high speed (e.g. 1 000 g) for about 2 minutes.
  • Discard the supernatant fluid and resuspend the cells in a further 5 ml of saline. Mix, centrifuge, and discard the supernatant fluid.                                        
  • Prepare a 3% red cell suspension by adding 1 volume of packed cells to 30 volumes of saline.

3. Antiglobulin polyspecific (Broad-spectrum) reagent, usually colour coded green. AHG reagent will agglutinate red cells sensitized with antibodies and/or coated with detectable levels of complement components.

4. Anti-D serum to make AHG control cells.

5. AHG control IgG sensitized red cells, prepared as follows:

  • Wash group O Rh positive red cells (obtained from a group O Rh positive person) three times in saline. Discard the final saline supernatant fluid.
  • Add an equal volume of IgG anti-D to the packed red cells and mix.
  • Incubate at 37 C for 30 minutes. Wash the cells four times in saline. Remove the final supernatant fluid.
  • Suspend the packed cells in saline to make a 5% red cell suspension. When added to AHG reagent, the sensitized cells should show visible agglutination
  • Store the sensitized cells at 2–8 C. They can be kept for 2–3 days.

INVESTIGATION OF HEMOLYTIC DISEASE OF NEWBORN (HDN)

1. Carry out ABO and Rh grouping of the mother and infant.
There can be no Rhesus incompatibility caused by anti-D antibody unless the mother is Rh negative and the infant is Rh positive.
occasionally, when grouping the infant’s
cells they may not appear Rh positive when antigen D receptors on the baby’s cells have been coated with maternal anti-D.


3.  Measure the infant’s haemoglobin and serum bilirubin


2. Carry out a DAT test on the infant’s cord cells. The DAT will be positive in Rhesus HDN.


4. Examine a Romanowsky stained blood film for the features of HDN, including spherocytosis which is usually less marked than in ABO HDN, polychromasia (reticulocytosis) and many nucleated red cells.

With Rhesus HDN, the infant’s haemoglo-
bin is usually below 140 g/l (14 g/dl) and the serum unconjugated bilirubin may rise to over 340 µmol/l (20 mg%). Such high levels of unconjugated bilirubin can cause irreversible brain damage (kernicterus).


5. Test also the mother’s serum for anti-D antibody when this has not been tested previously.

ABO GROUPING


ABO grouping consists of:

Cell grouping in which the red cells are tested for antigens A and B using anti-A and anti-B sera.
Serum grouping (reverse grouping) in which the serum is tested for anti-A and anti-B antibodies using known A and B red cells.

Why perform both cell and serum grouping ?

  • It greatly reduces the risk of errors in ABO grouping (serves as a double check).
  • There is less risk of misgrouping a group A person with weak antigen A as group O (or group AB as group B) because the error will be detected when serum grouping.
  • Errors due to autoagglutination will also be detected more easily.
  • Serum grouping using a tube will also detect the presence of anti-A and anti-B haemolysins in group O donor blood.

Grouping infants and elderly patients

Serum grouping is not performed when grouping infants below 4 months of age because naturally occurring anti-A and anti-B antibodies are only formed 3–4 months after birth.

When ABO grouping elderly people or persons with a gamma globulin deficiency, anti-A and anti-B may react weakly in the serum group and therefore cell grouping will be more reliable.

ANTIBODIES OF ABO BLOOD GROUP SYSTEM

In the ABO blood group system, naturally occurring IgM anti-A and anti-B are present in the serum in the absence of the corresponding red cell antigen.

Although described as naturally occurring (allo-) antibodies, anti-A and anti-B are not detectable in the blood of newborn infants. The antibodies become detectable at about 3 months of age following exposure to A and B like substances present in the environment e.g. in bacteria and some foods.

As a person gets older the concentration of naturally occurring anti-A and anti-B in the blood becomes less and these antibodies may be difficult to detect in the serum of some elderly patients.

Occasionally IgG hyperimmune anti-A and anti-B can be found in the serum of group O persons in response to stimulation by A and B like antigens in the environment, and following pregnancy, or the injection of some vaccines or toxoids. In tropical
countries it is common to find lytic IgG anti-A, anti-B, or both in the fresh serum of up to 50% group O persons. Lytic anti-A is also found in group B persons and lytic anti-B in group A persons (about
25% of sera).

Serious haemolytic reactions can occur when Group O whole blood containing anti-A and anti-B haemolysins is used to transfuse non-group O persons. Immune IgG lytic anti-A and anti-B can cross the placenta and cause ABO haemolytic
disease of the newborn (HDN).