Hello everyone, how are you all? i hope you all are doing well and i am too but i am mourning in my heart for one personal thing. I am posted in the department of transfusion medicine for this one month as a part of peripheral postings.I decided to learn as much as i can this month. Whatever i am learning i will post here and teach you. Lets dwell into it......
Today i am going to tell about Column Agglutination Test(CAT) also called Gel card test and how it is useful. We are going to see forward grouping.

This is a test used for blood grouping and cross-matching technique. The gel card has 6 to 8 columns. These columns have gel particles which act as filtering media. So it will filter non agglutinated red blood cells and it won't filter agglutinated red blood cells.
Here you can see that is gel card(microtube card) having 8 columns or microtubes.The first four it has labelling as A, B, AB and D(RH1). What are these? basically these columns or microtubes or filled with gel medium made up of dextran-acrylamide and first 4 has 4 colors like blue (A), Yellow(B) etc.. So first microtube contains gel medium and antisera(anti-A antibodies), second tube contains gel medium and anti-B antibodies, third tube contains anti-A and anti-B antibdoies and fourth tube contains anti-D antibodies and 5th column or microtube is neutral and there will be no antibody only it is gel.

Plz note - Its 1% red cell suspension not 5%
The above pale red color is red cell suspension which is made from patients blood that we have to test. so we will add 50 uL of patients red blood suspension in each microtube or column till the control microtube. Here we are adding patients red cell suspenion because we are testing patients red blood cells what they are having on their surface i mean what antigens they are having. So depending on which antigen is present on red blood cell surface we can tell which blood groups patient is having and for that we are using antibodies against to the antigens and checking whether agglutination or clumps is happening or not.

Here the above image represents first 5 microtubes(columns) of gel card and the blue color represents gel and red color represents antibodies present in it. so we will add 50 uL of 1% red cell suspension prepared from patients blood in each column. here red cells are represented in green color. After adding 50uL of 1% red cell suspension we have to centrifuge at 1000 rpm for 10 mins(it varies lab to lab)
So if antigen present red blood cell is A. In the first column these antigens on rbcs will react with anti-A antibody and form agglutination. In the same way for 2nd column if the rbcs if they have antigen B on its surface it reacts with anti-B and agglutination will form. The 3rd column which are having anti-A and anti-B antibodies reacts with antigen A and antigen B on the red blood cells if they have on its surface and form agglutination. Same for 4th column if they carry rh or D antigen on its surface they will react and form agglutination. 5th column won't react because no antibodies in it.
so after adding patient's 1% red cell suspension and centrifugation. the red cells which doesn't have respective antibodies will freely pass through and they get settles on the bottom. So thats why 5th column which is control and doesn't have any antibodies, the red cells will gets filtered through the gel and will be settled at bottom. If suppose red cells have antigen-A on its surface and they will form agglutination in the first and 3rd column because 1st and 3rd column has anti-A antibodies. So the agglutination or clumps will not pass through gel to the bottom and they will remain on top of gel or topper half of gel column or scattered through the gel column etc.... we can go for tube method which is gold standard if we have any doubt. No we can see few examples and you will understand

Here first see A- Agglutination happened is on top of the gel- So its Positive because Red cells have antigen A and agglutination(clumps) formed by the anti-A antibodies and remained on top didn't get filtered through gel.
and B- red cells are at bottom after centrifugation because they got filtered through gel. This is because red cells doesn't have antigen B on its surface to get agglutinated by anti-B in 2nd column, so they got filtered.
3rd column, red cells have antigen A so they got agglutinated because it has both anti-A and anti-B antibodies.
column 4, D antigen on red cell surface got agglutinated by anti-D antibody and remained on top of the gel
5th column is control where there are no antibodies and red cells will get settled down.
So here A, AB, D column got agglutinated. Column B(2nd column) didn't get agglutinated which is negative means B antigen is not present on its surface.
A antigen and D-antigen present and B-antigen not present. So, patients blood grouping is A positive (A+).

This procedure is called Forward grouping(Cell grouping). There is reverse grouping also with this gel card and we will see in our next post.
I will give you another gel card and you plz interpret it so that i will get to know whether you understood or not.

Interpret this Gel card

References

• Roback JD, et al. Technical Manual, 20th ed. AABB.
• Dacie & Lewis: Practical Haematology, 12th ed.
  Thanks for reading,
  with regards,