Most sinus infections are diagnosed based on your symptoms and how long they’ve lasted, not through imaging or lab tests. A doctor will ask about the type and duration of your symptoms, look inside your nose, and use established criteria to determine whether you have a sinus infection and whether it’s likely viral or bacterial. CT scans and other tools are reserved for complicated or chronic cases.
What Doctors Look for During Your Visit
A sinus infection diagnosis starts with your symptom profile. Clinicians use a standardized checklist that divides symptoms into two tiers. The major symptoms are thick or discolored nasal discharge, nasal congestion or obstruction, facial pain or pressure, a feeling of fullness in the face, reduced sense of smell, and fever. Minor symptoms include headache, ear pain or pressure, bad breath, dental pain, cough, and fatigue.
To meet the diagnostic threshold, you generally need either two major symptoms or one major symptom plus at least two minor ones. Your doctor won’t necessarily announce they’re running through this checklist, but these are the criteria guiding their assessment.
The physical exam itself is relatively brief. Your doctor will look inside your nostrils for signs of swelling, redness, or pus-like drainage. They may press on your cheeks, forehead, or the area between your eyes to check for tenderness. Some doctors use a technique called transillumination, where a light is held against the sinuses to check for fluid buildup, though this method has limited reliability. Partial opacification (when the sinus isn’t completely blocked) is a nonspecific finding that doesn’t confirm much on its own.
Viral vs. Bacterial: The 10-Day Rule
This distinction matters because it determines whether antibiotics would help. The vast majority of sinus infections start as viral infections, and most resolve on their own. Your doctor uses timing and symptom patterns to figure out whether bacteria have likely taken over.
The most widely used standard comes from the American Academy of Otolaryngology: if your symptoms persist for at least 10 days without any improvement, a bacterial infection is the likely culprit. The key phrase is “without improvement.” If you’re slowly getting better, even after 10 days, the cause is probably still viral.
A second pattern doctors watch for is called “double sickening.” This is when you start to feel better after a few days of a cold, then suddenly get worse again with new or returning symptoms like thicker discharge, worsening facial pain, or a fresh fever. That reversal strongly suggests bacteria have moved in on top of the original viral infection.
A third scenario involves severe onset: high fever (above 102°F) paired with thick, discolored nasal discharge lasting at least three consecutive days. This pattern can indicate a bacterial infection even before the 10-day mark.
When Imaging Comes Into Play
If your sinus infection is straightforward, you won’t need a CT scan or any other imaging. Clinical judgment is sufficient for the majority of cases, and the treatments for uncomplicated sinusitis are well established. CT scans can actually be misleading in acute infections because findings are often nonspecific. Swelling that shows up on a scan might reflect a simple cold rather than a true sinus infection.
CT scans become useful in a few specific situations: when symptoms keep coming back, when sinusitis has lasted 12 weeks or more and a doctor needs to see the anatomy of your sinuses, or when surgery is being considered. They’re also used when complications are suspected, such as infection spreading to the tissue around the eye. In those cases, a contrast-enhanced scan may be ordered.
MRI is rarely used for sinus infections. Its main role is distinguishing between different types of soft tissue, such as when a fungal infection or a growth is suspected. For standard sinusitis, MRI has no advantage over CT, tends to produce more false positives, doesn’t image bone well, and costs more.
Nasal Endoscopy and Cultures
If your symptoms are chronic or don’t respond to initial treatment, an ENT specialist may perform a nasal endoscopy. This involves threading a thin, flexible tube with a camera into your nasal passages. It gives a direct view of the mucosa (the lining of your sinuses) and the key drainage pathways, particularly the middle meatus, which is the main exit route for three of your four paired sinus groups.
Endoscopy is often done before imaging because it can reveal the condition of the tissue in real time: swelling, polyps, pus draining from a specific sinus opening. If the specialist sees drainage, they can collect a sample directly from the source using a swab or suction trap. This targeted culture is far more accurate than a standard nasal swab, because it captures bacteria from the sinus itself rather than whatever happens to be living in the front of your nose.
How Chronic Sinusitis Is Diagnosed Differently
If your symptoms have lasted 12 consecutive weeks or longer, the diagnosis shifts from acute to chronic rhinosinusitis, and the bar for diagnosis gets higher. You need at least two of four cardinal symptoms: facial pain or pressure, reduced or lost sense of smell, nasal drainage, and nasal obstruction. But symptoms alone aren’t enough. Chronic sinusitis requires objective evidence, meaning your doctor needs to see physical signs on an exam (such as pus, swelling, or polyps in the middle meatus) or abnormalities on a sinus CT scan.
This two-part requirement exists because facial pressure and congestion lasting months can have other causes. Without visible inflammation or imaging confirmation, the symptoms may point to migraines, tension headaches, or other conditions instead.
Conditions That Mimic Sinus Infections
Sinusitis is frequently overdiagnosed, particularly as a cause of headaches. Pain in the forehead or around the cheeks and eyes feels like it must be sinus-related, but migraines are actually the more common source of pain in those areas. Migraines can even cause nasal congestion and a runny nose, which makes the overlap especially confusing.
To count as a true sinus headache, the pain needs to occur alongside other signs of sinus disease (like thick discharge or endoscopic findings), happen at the same time as those sinus symptoms, and resolve within seven days of the sinus infection improving. If your “sinus headaches” keep returning without other clear signs of infection, a migraine evaluation may be more productive.
Allergic rhinitis (hay fever) is another common mimic. Allergies cause congestion, facial pressure, and mucus production that can feel identical to a mild sinus infection. The distinguishing features are itchy eyes or nose, sneezing in clusters, clear (not discolored) discharge, and symptoms that follow a seasonal or environmental pattern. Chronic nasal congestion from allergies can sometimes lead to actual sinus infections by blocking normal drainage, so the two conditions can also overlap.
Diagnosis in Children
Kids get diagnosed using similar timing rules, but the symptom emphasis shifts. The American Academy of Pediatrics identifies three patterns that point to bacterial sinusitis in children ages 1 to 18. Persistent illness means nasal discharge of any quality, or a daytime cough, or both, lasting more than 10 days without getting better. A worsening course means symptoms start to improve and then get worse again, the same “double sickening” pattern seen in adults. Severe onset means a fever of 102.2°F or higher along with thick, discolored nasal discharge for at least three consecutive days.
One notable difference: in children, the quality of nasal discharge matters less than its duration. Clear, runny discharge that drags on for more than 10 days without improvement still qualifies, whereas in adults, thick or discolored drainage is given more diagnostic weight. Cough is also more prominently tracked in pediatric criteria, since young children are less able to describe facial pressure or reduced smell.