Immunologic testing should be performed when there is a clinical history of recurrent infections and fevers, either refractory to treatment, or caused by opportunistic organisms. The history of infections supports abnormal immunologic testing findings.Rechecking abnormal values in 30-45 days is advised to account for dynamic changes during disease in an immunocompetent system.

The type of organism isolated from the site of infection often indicates the areas of the immune system with potential deficiency.

It is important to rule out other causes that may predispose to infections, including conditions that disrupt pathogen clearance mechanisms (e.g. stress, immunosuppressive therapy, viral infections, catheterizations, allergies, auto-immunity, lymphoma/lymphosarcoma).

Immunologic testing in foals is better interpreted when accompanied by control samples from age-matched and breed-matched healthy foals to account for developmental changes in the immune system. Yet, the diagnosis of immunodeficiency in the young is difficult before 3 months of life, and often requires repeated testing to measure trends in immune competence.