Coomb’s test is an immunological test that is used to assess the presence of antibodies against red blood cells. There are two variations of the Coomb’s test: the indirect Coomb’s test and the direct Coomb’s test. Each test serves its own purpose in the clinical setting. 

The basic idea is to identify the presence of antibodies against red blood cells because the presence of such antibodies can cause the lysis of those RBCs. But why are we interested in such antibodies in the first place? 

There are two clinical scenarios in which identifying the presence of antibodies against RBCs is useful. The first, is in the setting of autoimmune haemolytic anemia. The second, is in the setting of blood transfusion and pregnancy. Let’s look at each of these scenarios separately. 

In a setting of haemolytic anaemia, we as clinicians want to know if antibodies against RBC’s are present or not. If such antibodies are present then we must find a way to remove them so that hemolysis does not continue. But how do we identify these antibodies? This is where the Coomb’s reagent comes into play. 

The Coomb’s reagent is prepared by injecting human serum into animals. This leads to the animal forming antibodies against the human antibodies. This means that these antibodies can identify and bind to antibodies that bind to human RBCs.

Now in autoimmune haemolytic anaemia, the RBC’s have antibodies bound to them. When the Coomb’s reagent is added, it binds to the already formed antibody-RBC complex. This leads to agglutination which can be observed by the naked eye. This is a positive direct Coomb’s test. 

So the purpose of the direct Coomb’s test is to detect anti-RBC antibodies bound to RBCs pointing toward autoimmune hemolytic anemia. 

Now let us consider the second scenario. In case of blood transfusion, we want to know if the recipient has antibodies against the donors blood or not. So we collect the recipients blood and extract the serum. To this serum, we add a few drops of the donors blood. If the antibodies in the recipients blood attach themselves to the donor’s RBCs then we can add Coomb’s reagent and identify the antibody RBC complex. This would mean that they cannot transfuse that particular donor’s blood to that particular recipient. If the agglutination does not happen, it means that the antibodies in the recipient’s serum did not attack the donors RBCs and hence a transfusion can take place. 

This method of identifying free floating antibodies against RBC’s is known as indirect Coomb’s test. 



Another scenario to consider is the Rh negative mother who becomes pregnant. In the first pregnancy, if the baby is also Rh negative, then there is no problem. But if the father is Rh positive then the baby may be Rh positive. In that case, during the delivery, there is minimal exchange of blood between the mother and child. This causes the mother to be “sensitized,” to the baby. This is just a fancy way of saying that the mother develops antibodies to Rh positive RBCs. This antibodies are called D antibodies. 

Now in the second pregnancy, if the baby is Rh negative, again there is no problem. But if the baby is Rh positive, then the Anti D IgG antibodies will cross the placenta and bind to the fetus’ RBCs and cause hemolysis. So the only way to prevent this is to identify the presence of this D antibody in the mother and neutralise it. So the serum of the mother is taken, and a little bit of the baby’s blood (or any Rh positive blood) is added to it. The Coomb’s reagent is added to this to see if any antibody RBC complexes have formed. If they have, it means that the mother has antibodies against Rh positive RBCs. She must be must be desensitised. If not, then no active intervention is needed. 

The explanation of Coomb’s test is sound but complicated. Sometimes it gets confusing. When do you do the direct test? When do you do the indirect test? What are you looking for? 

Remember this. 

If you want to find antibodies bound to RBCs, you use the direct Coomb’s test and the sample you need is the venous blood (specifically washed RBCs. If you want to find free floating antibodies then you want to use the indirect Coomb’s test and the sample you need the patient’s serum.