Diagnosing Acute Bacterial vs Viral Rhinosinusitis

Lilian White, MD
Posted on February 17, 2025

Note: Special mention to Dr. Mark Ebell, family physician and deputy editor for evidence-based medicine at American Family Physician, who has authored several publications and studies that have advanced this topic.

Each year, more than 30 million diagnoses of acute rhinosinusitis are made in the United States.; it is most prevalent in patients 45 to 74 years of age. Antibiotics are prescribed in more than 80% of patients with acute rhinosinusitis, but studies demonstrate that only one-third of cases are due to bacterial infections.

Acute bacterial rhinosinusitis is generally a clinical diagnosis, making an understanding of distinguishing signs and symptoms critical for patient care. Preferred testing, such as aspirate of sinuses, is invasive and potentially harmful, relegating its use largely to research purposes. Current clinical guidelines by the Infectious Disease Society of America for diagnosing acute bacterial rhinosinusitis include the presence of fever greater than 102ºF, unilateral facial pain or pressure, and purulent discharge with nasal obstruction for 3 or more days.

Assuming a pretest probability of acute bacterial rhinosinusitis of 33%, the sign with the highest positive likelihood ratio (+LR = 3.9) and lowest negative likelihood ratio (-LR = 0.33) is the physician’s clinical impression, making it the most helpful sign to distinguish between bacterial and viral etiologies. If present, this sign increases the probability of acute bacterial rhinosinusitis to 66%. If absent, it reduces the probability to only 14%.

The symptom with the highest +LR (4.3; increasing the probability to 68%) is cacosmia: the presence of a foul-smelling odor detected by the patient. However, the absence of this sign is not especially helpful. Pain in the teeth and purulent nasal discharge are additional findings that may increase the probability of bacterial sinusitis, with +LRs of 2.0 and 1.3, respectively. Despite what many patients believe, the color of rhinorrhea is not helpful in distinguishing bacterial from viral etiologies of acute rhinosinusitis.

C-reactive protein (CRP) and leukocyte esterase have both been studied as point-of-care (POC) tests for distinguishing bacterial and viral causes of acute rhinosinusitis. CRP is used in some countries as a POC test; however, POC CRP testing is not currently an option in the United States. A Cochrane review found a number needed to test of 9 patients to reduce unnecessary antibiotic prescriptions, with a CRP of greater than 2.0 mg/dL in adults being supportive of bacterial infection. Use of CRP testing does not appear to affect patient recovery or satisfaction with treatment. Leukocyte esterase testing by urine dipstick has been studied with some promising results; however, prospective validation studies are currently lacking, so it is not recommended for clinical use at this time.

Although a procalcitonin blood level has been found to be useful for distinguishing between viral and bacterial pneumonia, evidence for its use in distinguishing acute viral and bacterial rhinosinusitis is currently lacking.

Ultrasonography does not appear to be helpful in distinguishing between viral and bacterial etiologies of acute rhinosinusitis. Computed tomography scans likewise are unable to distinguish between viral and bacterial causes.

A couple of clinical prediction rules have been developed to help diagnose bacterial rhinosinusitis, but they require additional validation studies before recommending widespread use. Both rules use physician assessment, elevated CRP, purulent nasal discharge, and preceding upper respiratory tract infection to estimate the probability of acute bacterial rhinosinusitis.