Disclosed are probes, systems, and devices that can be used to assay the presence of a biomarker in the sinus of a patient. The probes, systems, and devices can be used to contact the sinus fluid inside the sinus of a patient with an immunoassay for the presence of a biomarker that is characteristic of sinusitis. The probes can therefore be used to determine the underlying cause of sinusitis in a patient. Also provided are methods of treating sinusitis by administering to a patient a therapeutic regimen selected in view of a biomarker detected in the sinus of the patient. As the biomarker is associated with one or more causative agents of sinusitis, a therapy effective to treat the underlying causative agent can be selected.
