Always consider serious underlying conditions, such as tumors and immunodeficiency states, in the workup of chronic sinusitis. In general, plain radiography has low sensitivity and specificity. CT scanning is considered the imaging standard for evaluation of chronic sinusitis. Routine blood cell counts and sedimentation rates are generally unhelpful; however, these may be elevated in patients with fever.
The cornerstone in the diagnostic workup of chronic sinusitis is the radiologic examination. Nasal endoscopy is recommended in most cases prior to obtaining imaging because it demonstrates the condition of the nasal mucosa and evaluates for purulent drainage.
Radiographic findings in individuals with chronic sinusitis may demonstrate osteoblastic response in the affected sinus walls, mucoperiosteal thickening, opacification of sinus cavity, and even reduction of cavity size. Younger children with persistent respiratory symptoms probably have significant abnormalities that are observable on sinus radiographs. These radiographs provide noninvasive and rapid evaluation of the lower third of the nasal cavity and of the maxillary, frontal, sphenoid, and posterior ethmoid sinuses. Unfortunately, these views provide only limited information about anterior ethmoid anatomy and may be misleading in soft-tissue inflammatory disease; hence, more physicians are using CT for preoperative evaluation and MRI for excluding orbital and intracranial extension. [23, 25]
Unilateral sinus disease usually represents chronic inflammation but may be a sign of underlying malignancy, especially in individuals with a unilateral polyp. 
For more information, see the Medscape Reference article Imaging in Sinusitis.
Lund-Mackay scale for evaluation of images
Various staging systems have been proposed; however, no one system is accepted as the standard for use in chronic rhinosinusitis (CRS). Many studies use the Lund-Mackay scale to evaluate radiographic images. This scale grades the right and left sides independently, looking at the maxillary, anterior ethmoids, posterior ethmoids, sphenoid, and frontal sinuses, as well as the ostiomeatal complex. Each sinus is scored a 0 (no abnormality), 1 (partial opacification), or 2 (total opacification), while the ostiomeatal complex is scored either a 0 or 2 (for presence or absence of disease). Scores range from 0-24.
Establishing the presence of sinus infection requires obtaining bacterial and fungal cultures. These can be obtained directly from the sinus cavity (by maxillary sinus tap or during surgery) or endoscopically from the ostia. Studies of chronic sinusitis have demonstrated no correlation between nasal flora and culture from the sinuses. Nasal swab cultures have therefore no diagnostic value. In severe cases, blood cultures, including fungal blood cultures, may be helpful.
Maxillary sinus tap
Traditionally, maxillary sinus tap via inferior meatal puncture was performed for sinus culture. Many otolaryngologists have moved away from maxillary sinus tap because of the discomfort of the procedure and the understanding that a culture of an organism from the middle meatus may be more accurate to determine the bacteria involved in the disease process.
Endoscopically directed middle meatal culture
Recent literature has supported the use of endoscopically directed culture of the middle meatus (the primary drainage system of the anterior ethmoid, maxillary, and frontal sinuses) with the use of either a suction trap or a swab. Endoscopically directed middle meatal cultures had a sensitivity of 80.9% and a specificity of 90.5% in a recent meta-analysis. 
Plain radiography may show mucosal thickenings or sinus opacities. However, it is not adequate to diagnose CRS because abnormalities detected on plain films are not sensitive or specific for sinusitis. Air fluid levels are uncommon in chronic sinusitis. Ethmoid sinuses and the ostiomeatal complex are not visualized well on plain sinus radiography. For more information, see the Medscape Reference article Imaging in Sinusitis.
Multiplanar sinus CT scan is the preferred imaging technique for evaluating CRS. Sinusitis is characterized by the presence of sinus mucosal thickening, sinus ostial obstruction, and sinus opacification. Other findings include polyps, mucoceles, and bony changes due to CRS (sclerosis, septations, erosions, and bowing).
Contrast-enhanced CT scanning is the current radiologic criterion standard for the evaluation of sinus diseases, although performing CT scanning in all patients with chronic sinus disease may be prohibitively expensive or medically unnecessary. CT scans are usually indicated after failure of maximal medical therapy, before surgical planning for evaluation of suspected complications, and when a neoplasm is a possibility. CT scan combined with endoscopic examination helps the surgeon to make operative decisions.
Coronal CT scan of the sinus correlates best with the surgical approach, permitting visualization of the anatomy of the nasal cavity, ostiomeatal complex, sinus cavities, and surrounding structures such as the orbit, cribriform plate, and optic canal. Anatomic obstructions at the ostiomeatal complex and dental pathologies are visualized well. Specific entities in the sinus cavity, such as aspergilloma, are also visualized well.
Most centers now offer limited sinus CT scans that consist of 5-12 coronal cuts. These limited or screening CT scans cost about the same as a plain radiography but provide more information.
Magnetic resonance imaging (MRI) is generally reserved only for complex cases. Soft-tissue contrast is better with MRI. Neoplasms, orbital and intracranial complications, and fungal sinusitis can be better evaluated with MRI.
Biopsy samples from the maxillary sinus mucosa of patients with chronic sinusitis show basement membrane thickening, atypical gland formation, goblet cell hyperplasia, mononuclear cell infiltration, and subepithelial edema. The mononuclear cell infiltrate often predominantly demonstrates neutrophils in acute disease and eosinophils in chronic disease. Rarely, squamous cell metaplasia may be seen.
Brush biopsy or turbinate biopsy
Evaluation of cilia function with a brush biopsy or turbinate biopsy can be considered in cases of presumed cilia dysfunction.
Specimens obtained from sinus openings via endoscopy correlate well with those obtained with endoscopic surgery or sinus puncture. These should be processed for cultivation of aerobic and anaerobic bacteria, as well as fungi. Specimens evaluated for anaerobic bacteria should be sent in proper transport media. Liquid specimens are preferred to swab specimens.
Environmental allergen evaluation should be considered. Radioallergosorbent assay test (RAST) or skin testing for allergens may play an important role in treating patients with chronic rhinosinusitis (CRS) and confounding allergies. Perform allergy testing if allergy is thought to be the underlying cause.
Associated immune deficiency is evaluated with serum immunoglobulin and IgG subclass determination, antibody response to specific antigens, and HIV antibody testing (when indicated).
A sweat test for cystic fibrosis should be considered in all children with nasal polyposis and CRS.
Total immunoglobulin E (IgE) levels, as well as the degree of staining of IgE in sinus epithelium and subepithelium, can be tested and may be helpful to evaluate for allergic fungal sinusitis. 
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