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A physician may be able to diagnose whether a patient has sinusitis and what may be causing the inflammation. An accurate diagnosis is crucial because different types of sinusitis require different treatments. However, this can be difficult because the symptoms of sinusitis are often similar to those of colds and allergies.

A physical examination, patient medical history and list of symptoms are important components necessary for a diagnosis to be made. A physician will look for tenderness over the sinus cavities, swelling of the mucous glands, nasal secretions, postnasal drip and swelling around the eyes. A physician will also likely ask about any recent colds or history of allergies, and whether a patient smokes. A sample of nasal discharge may be collected and tested for the presence of bacteria.

A sinus x-ray, magnetic resonance imaging (MRI) or computed axial tomography (CAT) scan may help better define the extent of sinusitis in a patient and the degree of blockage present. A nasal endoscopy, called a rhinoscopy, uses an endoscope (thin, flexible tube and camera) to help a physician examine the area where the sinuses and middle ear drain into the nose.

Special tests known as cultures may be performed to identify the presence of bacteria or fungi. Biopsies of the cells lining the nasal cavity may also reveal important information. Blood tests may be performed to rule out conditions associated with sinusitis, such as immune system deficiencies or cystic fibrosis (a hereditary disorder affecting the lungs and digestive tract).

If allergies are a suspected cause of sinusitis, allergy skin testing or other diagnostic tests may be used to identify the type of allergies that may be triggering symptoms.