Direct Coombs test (antiglobulin test) in cats and dogs

What it shows

In IMHA the immune system coats red blood cells with antibodies (IgG, IgM) or with complement (C3), a protein of the immune defence. Coated cells are removed early or burst, and the animal becomes anemic.

In the lab the red cells are washed free of plasma and mixed with a reagent that binds to these antibodies and to complement. If enough of them sit on the cells, the reagent links neighbouring cells and they clump (agglutinate). The test is called “direct” because it looks at antibodies already stuck to the cells. The reagent has to match the species, so the test exists for dogs, cats and horses.

Higher than reference — what it may be a sign of

• Positive result in an anemic animal — supports immune destruction of red cells when other findings fit: signs of new red cell production (polychromasia, raised reticulocytes), spherocytes on the smear, red cells clumping on their own (autoagglutination) and raised bilirubin.
• Primary or secondary — the test cannot tell whether the immune reaction has no identifiable trigger (primary) or is set off by something else: infection, inflammation, cancer, some drugs or vaccination. Positive results are described with Babesia in dogs and with haemotropic mycoplasma in cats.
• Weak positive — does not always mean the immune system is the main cause of the anemia; it is also seen in some cancers.

Positive results also occur in animals without IMHA, which is why the vet checks the complete blood count and chemistry for supporting evidence. A definite diagnosis of IMHA usually needs at least two signs of immune destruction — spherocytes, true autoagglutination or a positive Coombs test. When spherocytes and true autoagglutination are both clearly present, the Coombs test may not be needed at all.

Lower than reference

A negative result makes IMHA less likely but does not rule it out. About 10–30% of dogs with IMHA test negative: red cells can be destroyed with as few as 20–30 antibody molecules on a cell, while the test needs more than 200–300 to turn positive. Other reasons for a false negative are antibodies that bind weakly and are washed off, antibodies lost from the cells during storage, and a reagent so concentrated that it blocks clumping (the prozone effect) — labs dilute the reagent to avoid this. With many spherocytes or clumping that persists after washing, a negative Coombs test does not exclude IMHA.

Whether recent steroid medication makes the test negative is not settled: an owner guide lists it as a cause, while a study in dogs found no reliable effect of steroids or of a transfusion in the previous 21 days.

Look at together with

• Hematocrit — how deep the anemia is.
• Smear: spherocytes, autoagglutination, polychromasia — other signs of immune destruction and regeneration.
• Reticulocytes — whether the anemia is regenerative.
• Bilirubin — rises when many red cells are broken down.
• Infection tests — Babesia, haemoplasma and others, when a secondary cause is looked for.

Units and conversion

The test has no units. The form gives a conclusion: positive or negative. Some methods test the cells against a series of reagent dilutions (one laboratory uses 1:2 to 1:2048), but the conclusion is still positive or negative. In the app the result is stored as text.

Reference range

The reference range is on your lab’s form; for this test the expected result is negative. In a study of dogs, all non-anemic dogs tested negative. A positive result is never read on its own: the vet weighs it together with the degree of anemia, the smear and agglutination, and confirms IMHA with more than one finding before deciding what to do next.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Coombs test — hematology / tests.
2. eClinPath (Cornell University College of Veterinary Medicine). A positive Coombs test.
3. VCA Animal Hospitals. Coombs’ Test.
4. Merck Veterinary Manual, professional version. Regenerative Anemias in Animals.
5. eClinPath (Cornell University College of Veterinary Medicine). Shape changes (poikilocytosis) — spherocytes.