Which lab finding supports a diagnosis of allergic rhinitis?

A positive reaction for a scratch test is a key indicator for allergic rhinitis because this test assesses the body's immune response to specific allergens. When an individual has allergic rhinitis, their immune system reacts to environmental substances—such as pollen, dust mites, or pet dander—by producing immunoglobulin E (IgE) antibodies. The scratch test involves introducing small amounts of potential allergens into the skin and observing for an allergic response, which manifests as localized redness and swelling.

This reaction confirms the presence of an allergy, reinforcing the diagnosis of allergic rhinitis. In contrast, the other options refer to different conditions or immune responses not directly tied to allergic rhinitis. For example, an elevated eosinophil count may indicate various allergic diseases or parasitic infections but cannot definitively diagnose allergic rhinitis on its own. Neutrophils are associated with bacterial infections and inflammatory responses, while a positive Mantoux test indicates exposure to tuberculosis rather than an allergic process. Thus, the positive reaction for the scratch test serves as the most relevant finding to support a diagnosis of allergic rhinitis.