Sinus infections are diagnosed primarily by symptoms and how long they’ve lasted, not by imaging or lab tests. Most of the time, a doctor can determine whether you have a bacterial sinus infection based on three specific patterns: symptoms lasting 10 or more days without improvement, symptoms that get better then suddenly worsen, or severe symptoms from the very start. No swab, blood test, or scan is needed in the majority of cases.
The Three Patterns That Point to Bacterial Infection
Almost every sinus infection starts as a regular cold. The critical question is whether bacteria have taken over, because that’s what determines whether antibiotics would help. Doctors use three distinct patterns to make that call.
Persistent symptoms beyond 10 days. If you still have nasal congestion, colored discharge, or a daytime cough after 10 days with no sign of improvement, that crosses the threshold from a viral cold into likely bacterial sinusitis. A typical cold improves on its own within 7 to 10 days. Symptoms that plateau or slowly worsen past that window suggest bacteria have colonized the inflamed sinuses.
The “double sickening” pattern. You start with a cold, begin to feel better around day 3 or 4, then get noticeably worse again between days 5 and 10. New fever, a sudden increase in nasal discharge, or worsening cough after that initial improvement is a hallmark of bacterial infection setting in on top of a viral illness.
Severe onset. A fever of 102.2°F (39°C) or higher paired with thick, discolored nasal discharge for at least three consecutive days suggests a bacterial infection from the start, without the usual viral warm-up period.
If your symptoms don’t fit any of these three patterns, you most likely have a viral sinus infection that will resolve on its own.
What Happens During the Physical Exam
When you visit a doctor for suspected sinusitis, the exam is relatively straightforward. Your doctor will look inside your nose using an anterior rhinoscopy, checking the condition of the nasal lining and whether there’s colored discharge. Swollen, reddened tissue and thick mucus draining from the area between the sinuses and the nasal passage support the diagnosis.
You might expect your doctor to press on your cheeks or forehead to check for tenderness. While facial tenderness to touch is common with sinus infections, studies show that palpation and transillumination (shining a light through the sinuses) have little predictive value on their own. These techniques can’t reliably confirm or rule out infection. If your doctor suspects something more complex, they may use a nasal endoscope, a thin flexible camera that can see exactly where purulent discharge is originating and whether there’s a structural blockage.
When Imaging Is Actually Needed
For a straightforward sinus infection, you don’t need a CT scan or X-ray. The American Academy of Otolaryngology explicitly recommends against imaging for uncomplicated acute sinusitis. CT scans can’t reliably distinguish bacterial infections from viral ones, and abnormal findings on imaging are extremely common even in people without symptoms.
Imaging becomes important in a few specific situations. If your doctor suspects the infection has spread beyond the sinuses, into the eye socket or toward the brain, a contrast-enhanced CT scan can identify complications like orbital cellulitis or abscess formation. Signs that would trigger this concern include swelling around the eye, vision changes, severe headache with high fever, or altered mental status.
CT scans are also the standard for evaluating chronic sinusitis and for surgical planning. A non-contrast CT provides detailed images of the bony sinus anatomy, helping surgeons identify structural variations before operating. For recurrent or chronic cases, this imaging confirms the diagnosis and maps the extent of disease.
Diagnosing Sinus Infections in Children
Children get sinus infections frequently, and the diagnostic criteria from the American Academy of Pediatrics mirror the adult patterns with a few important nuances. Persistent illness is the most common presentation: a runny nose of any quality (clear or colored) or daytime cough lasting more than 10 days without improvement. In children, nasal discharge doesn’t have to be thick or green to count. Clear, watery discharge that simply won’t quit is enough when it persists past the 10-day mark.
The worsening pattern in children is defined as a cold that starts improving, then comes back with new-onset fever (100.4°F or higher), a substantial increase in cough, or worsening runny nose. Severe onset follows the same criteria as adults: fever of 102.2°F or higher with thick, colored discharge for at least three consecutive days. Because young children can’t describe facial pressure or loss of smell, doctors rely more heavily on observable symptoms like cough, discharge, and fever.
Acute vs. Chronic: Different Diagnostic Standards
Acute sinusitis lasts less than four weeks and is diagnosed by symptoms alone. Chronic rhinosinusitis is a different condition with a higher bar for diagnosis. To qualify, you need at least two of four cardinal symptoms, facial pain or pressure, reduced or lost sense of smell, nasal drainage, and nasal obstruction, lasting for at least 12 consecutive weeks.
The key difference is that chronic sinusitis also requires objective evidence. Your doctor needs to confirm inflammation through nasal endoscopy or a CT scan. Symptoms alone aren’t sufficient. A non-contrast CT is the preferred imaging study, showing the extent of mucosal thickening, polyps, or structural problems that perpetuate the cycle of inflammation.
What a Sinus Infection Is Not
Many people who believe they have recurring sinus infections actually have migraines. Research published in the journal Neurology found that a surprisingly high percentage of patients diagnosed with “sinus headache” actually met the criteria for migraine. The confusion happens because migraines frequently cause nasal congestion, watery eyes, and facial pressure, symptoms that feel identical to sinus disease.
The distinguishing features: true bacterial sinusitis produces thick, discolored nasal discharge and follows one of the three timing patterns described above. Migraines tend to cause throbbing pain, sensitivity to light or sound, and nausea. Nasal symptoms during a headache shouldn’t automatically point to a sinus diagnosis. If you get repeated “sinus headaches” that never produce significant nasal discharge and don’t respond to decongestants, a migraine evaluation is worth pursuing.