Coomb's test
Meaning of Coomb's test: A test that finds antibodies clinging invisibly to red cells, by adding a second antibody that clumps them into something visible.
Definition of Coomb's test
The Coombs test is a blood test that detects antibodies or complement proteins stuck to the surface of red blood cells, or antibodies floating in plasma that would attack red cells if they met them. A reagent containing antibody directed against human antibody is added, and if the target is present the red cells clump together visibly. That clumping is the positive result.
Why is a second antibody needed?
Because the antibodies being hunted are invisible on their own. When one coats a red cell it usually sits there doing nothing that can be seen, and the coated cells stay separate in the tube. Adding a reagent made of antibodies aimed at human antibody changes that. Each reagent molecule grips the antibody on one red cell with one arm and the antibody on a neighboring cell with the other, bridging them until the suspension clumps into visible grains. That clumping is the whole readout. No clumping means there was nothing there to bridge.
How do the direct and indirect versions differ?
| Version | What is tested | Question it answers |
|---|---|---|
| Direct | The person's own washed red cells | Are these cells already coated inside the body? |
| Indirect | Plasma mixed with reagent red cells | Does the plasma hold antibodies that would attack cells? |
The direct version is used where red cells are being destroyed: autoimmune breakdown, a mother's antibodies acting against a baby's cells, a delayed reaction to transfused blood, and destruction triggered by certain medicines. The indirect version screens plasma ahead of a blood transfusion and during pregnancy, and it forms one stage of a crossmatch.
What does a positive result mean?
Less on its own than it appears. A positive direct test shows that antibody or complement is sitting on the red cells. It does not show that the cells are actually being destroyed, and it turns up in a small number of healthy blood donors and in people taking particular medicines who have no anemia at all. It is read alongside the blood count and the markers of red cell breakdown. Two limits are worth stating plainly: the test examines red cells and not platelets, since antibodies against platelets are found by entirely different methods, and ordinary blood grouping does not use this reagent, because those antibodies clump cells unaided. The formal name is the antiglobulin test.
Also known as
Used in a sentence
A direct Coombs test was performed to investigate the falling hemoglobin.
Good to know
A positive direct test shows only that antibody or complement is bound to red cells and not that they are being destroyed, and the test addresses red cells alone rather than platelets, which require separate assays.