What screening test should be performed first in a young patient suspected of having an immune dysfunction disorder?

A complete blood count (CBC) and white cell differential serve as foundational screening tests when evaluating a young patient for an immune dysfunction disorder. This test provides critical and comprehensive information about the quantity and proportions of various blood cells, including red blood cells, white blood cells, and platelets.

In particular, the white cell differential breakdown shows the levels of different types of leukocytes (such as neutrophils, lymphocytes, monocytes, eosinophils, and basophils), which can indicate potential immune dysfunction. For instance, a reduced number of lymphocytes may suggest lymphopenia associated with certain immunodeficiency disorders, while an elevated eosinophil count might indicate an allergic reaction or parasitic infection.

Other testing options, such as chemotaxis assays or complement levels, are typically more specialized and may be warranted based on findings from a CBC. Bone marrow biopsy, while informative, is an invasive procedure and not the first-line approach unless there's a strong indication of hematological malignancies or bone marrow disorders. Thus, initiating the evaluation with a CBC and white cell differential establishes a broad diagnostic perspective and guides further testing as needed.