Coombs’ test

What is this test for?
Looks for antibodies that act against your red blood cells.

Types
Direct antiglobulin test; Indirect antiglobulin test

Direct Coombs test

The Concept

• IgG antibodies that specifically bind to antigens on the red blood cell (RBC) surface membrane
• Red blood cells (RBCs) become coated with IgG alloantibodies (an antibody that occurs naturally against foreign tissues from a person of the same species) and/or IgG autoantibodies (An antibody that reacts with the cells, tissues, or native proteins of the individual in which it is produced)
• Complement proteins may subsequently bind to the bound antibodies

So,
- Coombs test is used to detect these antibodies or complement protein
- Red blood cells are taken
- Coombs reagent (antihuman globulin) incubated with the red blood cells
- if there are antibodies or complement proteins bound to red blood cells, agglutination occurs

Indirect Coombs test
- Used in pregnant women, and to test blood in blood tranfusion
- Detects antibodies which are not bound to red blood cells of patients
- If agglutination occurs, the indirect Coombs test is positive.
- rarely used to diagnose a medical condition


Examples

Alloimmune hemolysis

o Rhesus hemolytic diseases of the newborn
o Systemic lupus erythematosus
o Evans' syndrome (antiplatelet antibodies and hemolytic antibodies)
o Etc

Drug-induced immune-mediated hemolysis

o Methyldopa (IgG mediated type II hypersensitivity)
o Penicillin (high dose)
o Quinidine
o Etc