Immune-Mediated Anaemia: Coombs Test and Saline Agglutination

Immune-mediated anaemia (IMHA) means a dog's or cat's own immune system is destroying its red blood cells. Two tests, the saline agglutination test and the Direct Coombs test, look for antibodies stuck to those cells. A positive result may point to IMHA, but a negative result does not always rule it out.

What goes wrong in the body in IMHA?

Red blood cells carry oxygen around the body. They live for a limited time, and the bone marrow keeps making new ones. Anaemia means there are too few red cells, or too little of the red pigment inside them, to carry enough oxygen.

In IMHA, antibodies attach to the animal's own red cells by mistake. Antibodies are proteins that normally mark germs for attack. Once the cells are coated, the body removes or breaks them down much faster than the marrow can replace them. The cause is sometimes unclear. In other pets it follows another illness, such as some blood parasite infections, so vets also look for a trigger.

Which signs may owners notice?

Signs can appear quickly. Look out for:

• pale gums or pale inner eyelids
• a yellow tint to the gums, the whites of the eyes or the skin, called jaundice, which appears when broken-down red cells release a yellow pigment faster than the liver can handle it
• tiredness, weakness, fast breathing or a poor appetite
• urine that looks dark orange or brown

A pale, weak pet needs a vet promptly. Do not wait for lab results before seeking care.

How does the saline agglutination test work?

This is a quick slide test. A drop of the pet's blood is mixed with a drop of saline, which is salt water, and the vet or lab looks for clumping. In healthy blood the cells spread out. When antibodies are gluing red cells together, grainy clumps stay even after the blood is diluted.

The salt water matters because red cells can also stack up like coins in some sick animals, and this looks like clumping. The stacks fall apart when diluted, but true antibody clumps do not. The Saline Agglutination Test (Auto-agglutination) uses a small EDTA blood sample. It is simple and fast, yet an animal with IMHA may show no clumping, so a negative slide does not close the question.

What does the Direct Coombs test add?

The Direct Coombs Test (DAT) looks for antibodies, or parts of the immune system called complement, on the surface of red cells. The cells are washed and mixed with a reagent that reacts with those antibodies. If the cells clump, antibodies are likely on them. This test can pick up coating that is too light to cause clumps on a slide.

It has limits too. A negative result does not rule out IMHA. A positive result can also be seen in animals with other illnesses, so it is not a final answer by itself. The lab confirms the method and the handling of the sample when you book.

Which other tests round out the picture?

A Complete Blood Count (CBC) shows how low the red cell count has fallen and also counts white cells and platelets. The vet or lab may also look at a stained blood smear under the microscope for unusual red cell shapes.

A Reticulocyte Count measures young red cells newly released from the marrow. It tells the vet whether the marrow is trying to replace the lost cells. A strong response and a weak response point in different directions, and a weak one can also mean the marrow has not yet had time to respond. The vet may also test for blood parasites and other infections.

A registered veterinarian reads all of these results with the pet's signs, history and examination. What to do next, including any treatment, is the vet's decision.

Read next

IMHA is only one cause of low red cells, and Anaemia in Pets: Which Blood Tests Help Find the Cause explains how vets sort through the others, such as blood loss, parasites and long-term illness. If you also look after dairy animals, Growing Heifer Checks Before First Breeding shows how a baseline blood count is used to spot trouble early in young stock on a farm.

Testing in Bhopal

Bring your pet's blood sample to C-200, C Sector, Shahpura, Bhopal, or ask the lab to collect it free anywhere in the city. Samples from other districts can arrive by courier, bus parcel or through your own vet. Message +91 88175 01118 on WhatsApp or call, and the lab confirms the method, sample needs and reporting time when you book.