Coomb's test is very helpful in making a diagnosis of hemolytic anemia.
Once a patient is found to have anemia, with raised bilirubin and raised retic count, hemolysis becomes the most likely possibility. In coomb's test is of two types.
1. Direct Coomb's test
2. Indirect Coomb's test.

Direct Coomb's test:

In case of direct Coomb's test, patient's RBC's are washed clear of plasma and are mixed with Coomb's serum (antihuman globulins), if agglutination occurs, it means the Direct Coomb's test is positive.
If the direct Coomb's test is positive it means the patient has autoimmune hemolytic anemia.

Indirect Coomb's test. 

In indirect Coomb's test, patient's serum is taken instead of RBCs and this serum is then mixed and incubated with RBCs with a known antigen if patient's serum contains antibodies against the RBCs antigens then antibody-antigens complexes are formed, then coombs serum is added to the sample, if agglutination occurs it means the indirect Coomb's test is positive.
indirect Coomb's test is used in blood cross-matching and also used in screening of a pregnant woman for the presence of antibodies that can cause hemolytic disease of the newborn.

Example 1:
Pregnant Woman screening

1. Serum from the blood sample of the pregnant woman is taken.
2. This serum is then mixed with the RBCs of the fetus.
3. Coomb's serum is added to the sample
4. If agglutination occurs, then the indirect Coomb's test is positive and the baby is at risk of developing the hemolytic disease of the new born.

Example 2.
Crossmatching

1. Serum from the blood sample of recipient is taken
2. This serum is then incubated with the RBCs of the donor.
3. Coomb's serum is added the sample
4. If agglutination occurs then it means indirect Coomb's test is positive and the donor's blood is unsuitable for the patient.

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