Just Ask the Expert: A few tips on diagnosing IMHA

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Sep 01, 2010

Dr. Garcia welcomes internal medicine questions from veterinarians and veterinary technicians.
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Q. What steps should veterinarians take to diagnose immune-mediated hemolytic anemia? For example, is a saline agglutination test sufficient to confirm a diagnosis and begin prednisone therapy?


Jennifer L. Garcia, DVM, DACVIM
A. Diagnosing immune-mediated hemolytic anemia (IMHA) in veterinary patients can sometimes be a little tricky. Typically, we look for a regenerative anemia, hyperbilirubinemia, and evidence of agglutination, spherocytosis, or both.

REGENERATION

Evidence of regeneration is best assessed by looking at a reticulocyte count as it is the most sensitive indicator of regeneration. However, in a couple of situations, the anemia may not be regenerative in IMHA. The first is a patient with acute anemia. The bone marrow may take a few days to gear up, so early in the disease, the patient may not yet have an appropriate bone marrow response. A second situation is immune-mediated disease directed against red blood cell (RBC) precursors, not just peripheral RBCs. These cases are not common but do result in a nonregenerative anemia.

HYPERBILIRUBINEMIA

Hyperbilirubinemia is seen secondary to RBC destruction and is a common finding in IMHA. A dog with a regenerative anemia and an elevated bilirubin concentration would be a prime suspect for hemolytic anemia.

AGGLUTINATION

Clumping (agglutination) of antibody-covered RBCs is a common finding in patients with IMHA. A rapid confirmation of this can be done in-house with a saline agglutination test (SAT). To perform this test, place one drop of EDTA blood on a slide, and add one drop of saline solution (two drops for a cat). The purpose of the saline is to disperse any rouleaux formation as well as inflammatory proteins that may cause pseudo-agglutination. Rock the slide gently back and forth for one to two minutes looking for evidence of clumping. This should be visible to the naked eye but occasionally may require a peek under the microscope.

The purpose of a Coombs test is to detect the presence of erythrocyte-bound immunoglobulin. If the SAT result is positive, you have already determined that these immunoglobulins are present so a Coombs test is unnecessary. If the SAT result is negative, consider performing a Coombs test, but remember that this test is not highly sensitive or specific, so it will still need to be evaluated in light of the other laboratory findings and clinical signs.

CONFIRMATION

In most cases, if you have a patient with a strongly regenerative anemia, an elevated bilirubin concentration, and a positive SAT result or spherocytosis, you have enough to diagnose IMHA and begin therapy.

Jennifer L. Garcia, DVM, DACVIM, is a veterinary internal medicine consultant in Houston, Texas.