Complete blood count is most helpful in diagnosing which disorder?

In the context of diagnosing immunodeficiency disorders, a complete blood count (CBC) provides valuable information regarding the cellular components of blood, particularly white blood cells (leukocytes). Common variable immunodeficiency (CVID) is characterized by low levels of immunoglobulins and may also show abnormalities in the white blood cell count, typically indicating reduced numbers of certain types of lymphocytes or impaired responses to infections.

A CBC is particularly useful in identifying variations in leukocyte counts which can hint at deficiencies in specific immune functions. Patients with CVID may have normal or slightly reduced lymphocyte counts, displaying an inability to generate adequate antibody responses despite having a normal number of B cells. This is what makes the CBC a practical tool in the early evaluation and suspicion of CVID.

In contrast, the other disorders mentioned either require more specific tests for diagnosis or do not primarily manifest in the CBC results. For example, chronic granulomatous disease and leukocyte adhesion deficiency, while related to immune function, usually necessitate more specific tests like flow cytometry to assess the functionality of neutrophils or leukocyte adhesion molecules. Severe combined immunodeficiency generally presents with extremely low lymphocyte numbers, but the CBC alone may not denote the full extent of the condition