Although diagnosis is based on the presence of characteristic symptoms, some diagnostic procedures are performed, not only to confirm objectively referred symptoms, but also to define the severity of the disease and check for the predisposing factors. Usually, nasal endoscopy, CT-scan and allergy testing are performed. However, in cases that are not responsive to standard treatment, a lot of other procedures can be applied, including microbiology testing, cytology, measurement of secretion clearance, even biopsies of the sinus mucosa. When a main predisposing factor is recognized and treated, CRS severity is usually improved.

It is important to note that tumours, benign and malignant, have symptoms that would fit into diagnosis of CRS. If symptoms are dominantly on one side, if there are signs of bleeding on one side, and problems with vision and severe headaches occur, further diagnostic procedure, primarily detailed endoscopy and imaging with CT or magnetic resonance should be done.