Direct and indirect Coombs test !
- Topic
The Coombs test looks for antibodies against red blood cells: stuck to them (direct) or free in the plasma (indirect).
Updated 2026-11-01
What it is
The antiglobulin test uses an anti-human immunoglobulin (and anti-C3) reagent that agglutinates antibody-coated red cells. The direct test studies the patient's red cells and confirms that hemolysis is immune; the indirect test looks in serum for antibodies able to attack other red cells. It is basic in hemolytic anemia, transfusion and prenatal care.
Direct versus indirect Coombs
| Feature | Direct | Indirect |
|---|---|---|
| What is tested | The patient's red cells | The patient's serum or plasma |
| What it detects | Antibodies or complement already bound to red cells | Free antibodies against red-cell antigens |
| How | Antiglobulin is added to washed red cells | Serum is incubated with reagent or donor red cells, then antiglobulin is added |
| Classic uses | Autoimmune hemolytic anemia, hemolytic disease of the newborn, hemolytic transfusion reaction, drug-induced hemolysis | Antibody screen and crossmatch before transfusion; anti-D screening in pregnancy |
Clinical notes
- Rh-negative pregnant woman: the indirect Coombs detects anti-D; anti-D immune globulin prevents sensitization.
- Newborn with early jaundice: a positive direct Coombs points to hemolytic disease from Rh or ABO incompatibility.
In the lab
- Positive = agglutination; negative = no detectable antibodies against red cells.
- Warm autoimmune hemolytic anemia: IgG on the red cell; cold agglutinin disease: mainly C3.
- It can be positive in lupus, chronic lymphocytic leukemia, mononucleosis, Mycoplasma infection and syphilis, and sometimes with no clear cause in older adults.
Test yourself
Think of the answer, then tap the question to see it.
What does the direct Coombs test detect?
Antibodies (or complement) already bound to the surface of the patient's red cells.
Which test is used to look for anti-D in an Rh-negative pregnant woman?
The indirect Coombs, which detects free antibodies in maternal serum.
What direct Coombs pattern is expected in cold agglutinin hemolytic anemia?
Positivity mainly for complement (C3), with little or no IgG.
Which Coombs test is used in the crossmatch before transfusion?
The indirect one: the recipient's serum is tested against the donor's red cells.
Related
- Passive immunization: immunoglobulins and antitoxins
- Serology: IgM vs IgG, seroconversion, avidity
- Immunofluorescence and the ANA test
- Type II hypersensitivity (antibody against cells)
- ABO and Rh blood groups, hemolytic disease of the newborn
- Transfusion reactions
Related conditions
Microorganisms
Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.
Sources: Coombs test (MedlinePlus (U.S. National Library of Medicine), 2024); Red Blood Cell Antibody Screen (MedlinePlus (U.S. National Library of Medicine), 2024).
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Representative model, for educational purposes only.