The Indirect Coombs Test

In the last post we decided that the direct Coombs test was a way to identify patients suffering from an autoimmune haemolysis of their red blood cells - a positive test involved adding antihuman antibodies to a sample of the patient's blood and watching for agglutination.

The indirect Coombs test (often conducted before a patient receives a blood transfusion) aims to see if a patient's serum contains antibodies directed towards the a particular sample of blood.  A sample of the patient's serum is taken and added to some of the transfusion blood.  If the above antibodies are present, they will now have an opportunity to bind to the red cells.  This is tested for in exactly the same method as before: add antihuman globulin and see if the blood aggultinates.

The main differences between the direct and indirect tests are:

• The direct test always identifies immune destruction of the patient's own red cells, whereas the indirect test can look for potential immune destruction of another person's red cells (if the patient's serum ever comes into contact with them). 
• The direct test aims to see if the patient has autoimmune haemolysis, whereas the indirect test checks whether the patient will destroy a particular red blood set.
  

One again, a helpful diagram is the one below.  Click on it to view a bigger version.