The direct antiglobulin test (also called the Coombs’ test, or the DAT) is an important one for you to know. It’s used mostly in one particular setting: when you have a patient with a hemolytic anemia (one in which the red cells are getting busted open) and you want to know if the hemolysis is immune-related or not. As explained below, the DAT is positive in immune hemolytic anemias and negative in non-immune hemolytic anemias.
The whole point of the DAT is to find out whether there are antibodies and/or complement bound to the surface of the patient’s red cells. In an immune hemolytic anemia, the patient may have antibody, or complement, or both bound to his or her red cells. Since you can’t see the antibody or complement under the microscope, you need a way to determine whether these molecules are present – and that’s what the DAT is for.
Here is how it is done. A small amount of a reagent called Coombs’ reagent, or anti-human globulin (AHG) is added to the patient’s blood in a test tube. This reagent (depicted as blue antibodies in the diagram above) consists of antibodies directed against human antibodies. These antibodies are raised by injecting human antibodies into another animal (a rabbit, or a mouse, or some other non-human), and then collecting the anti-human-antibody antibodies the animal makes (the animal sees the human antibodies as foreign substances, and it makes its own antibodies against them). You also add some antibodies directed against complement to the patient’s blood sample (these are not depicted above).
The cool thing about the Coombs’ reagent is that if the patient’s red cells are coated with IgG, the Coombs’ reagent will bind to this IgG on the red cells, bridging the gap between adjacent red cells, and causing the red cells to clump together (see the right hand side of the diagram above)! You can see this clumping with the naked eye. The same principle works for the anti-complement antibodies; if there is complement bound to the red cells, the anti-complement antibody will bind to it, and the red cells will clump together.
So: if you see clumping in the test tube, the DAT is positive, and that means your patient has an autoimmune hemolytic anemia. The next thing to figure out is which kind of autoimmune hemolytic anemia it is – but that’s the subject of another post.
Note: The very nice depiction of the DAT above was created by A. Rad, and can be found on Wikimedia commons at: http://commons.wikimedia.org/wiki/File:Coombs_test_schematic.png.
We need your help! Pathology Student is completely independent and viewer-funded. If you find us useful, please consider donating whatever feels right to you. Every bit helps!You can donate here.
- Salihu Ahmad yoffo said Very interested page
- Kristine said Thanks, Michael, and welcome! So glad you liked the discussion of the RDW. No – I haven’...
- Michael A Martin said Hello Dr Kristine Krafts, I just joined your website. I enjoyed your discussion of the RDW. Are you...
- Ehsan said Hi Dr. Krafts. Just to say I love this post. Tanx.
- Kristine said Hi Deborah – Most any bacterial infection can cause a left shift. In general, as you allude to...
- Kristine said Thanks! So glad you found the post useful!
- deborah said My 2 year old Grandson has a 12,000 WBC with a left shift. He has been vomiting with some bile showi...
- derald madson, Jr. said Im a vascular surgeon in Ohio. im reviewing for my 10 year recertification boards. Nice job!
- Lauren Sherman said Thank you!
- AKHTAR HUSSAIN said hi mam I want to ask you one question I am very keen to know or more experience to read blood morpho...
- Taylor said Thanks so much for making this course! I’m a 4th year med student going into Pathology and thi...
- ogechi said This makes a lot of sense. Thanks for this explanation Dr Krafts.