How is blood cross matching performed?

Answer 1

A test tube of the patient's blood is sent to the blood bank and spun down, so that the red cells are in the bottom and the plasma is on top. The plasma is separated into a separate tube, and then a few drops of red cells are placed into another tube.

The red cells are washed first, which means that they are mixed with saline and spun in a centrofuge, and then the saline is removed. This gets rid of excess proteins which might cause a false reaction. Once the cells are washed, more saline is added to make a .8% suspension. The following test tubes are set up:

1. One drop of the red cell suspension + 2 drops of Anti-A antiserum

2. One drop of the red cell suspension + 2 drops of Anti-B antiserum

3. One drop of the red cell suspension + 2 drops of Anti-D antiserum

4. Two drops of patient serum + 1 drop of red cells with A antigen

5. Two drops of patient serum + 1 drop of red cells with B antigen

Each tube is mixed gently, then spun for 15 seconds in a centrofuge, then checked to see if the cells in the tube can be easily resuspended (negative) or if they are all stuck together in a clump (a positive reaction).

This is done with each cross-match to confirm the patient's ABO and Rh blood types, although, if the patient has had their blood typed in the lab previously, the lab worker may choose to only do the first three or the last three tubes just to double-check that the results the first time are OK.

An antibody screen is set up, which can take place either in a test tube or a special gel card. Reagent red cells are added to the tube or the card, and these cells are already known to contain a variety of antigens. The patient serum is added to the cells, and a reaction will be seen if the patient has antibodies.

As for the cross-match itself; On each bag of donor blood, there is a long tube of blood that is clamped at intervals to make sections. A section is removed, the blood poured into a labelled test tube, and the blood is diluted to a .8% suspension. 2 drops of the donor blood are put into another labelled test tube with 4 drops of the patient serum, and this is mixed and then spun for 15 seconds in the centrofuge. Compatible blood will be easily suspended, incompatible blood will be stuck together in a clump. The tube is checked microscopically to make sure that there aren't tiny clumps of cells.