When is primary immunodeficiency suspected?

Sometimes the signs and symptoms of primary immunodeficiency (PI) are so severe, or so characteristic, that the diagnosis is obvious. In most cases, it is not clear if a long string of illnesses are just “ordinary” infections, or if they are the result of an immunodeficiency.
Many conditions can produce an immunodeficiency, at least temporarily, and most children who seem to have “too many” infections are not, in fact, suffering from an immunodeficiency. Experts estimate that half of the children who see a doctor for frequent infections are normal. Another 30% may have allergies, and 10% have some other type of serious disorder. Just 10% turn out to have a primary or secondary immunodeficiency.

How is PI diagnosed?
What screening tests are used?

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