Most sinus infections are diagnosed based on your symptoms and how long they’ve lasted, not through lab tests or imaging. A doctor will ask about your symptoms, look inside your nose, and use specific timing rules to determine whether you’re dealing with a viral cold that will clear on its own or a bacterial infection that needs treatment.
The Symptom-Based Diagnosis
The primary “test” for a sinus infection is actually a careful review of your symptom timeline. Doctors use three distinct patterns to distinguish a bacterial sinus infection from a regular cold:
- Persistent symptoms: Congestion, facial pressure, or nasal discharge lasting 10 days or more with no improvement. A typical cold improves within 7 to 10 days, so symptoms that plateau or hang on past that window suggest bacteria have moved in.
- Severe onset: A high fever (102°F or higher) combined with thick, discolored nasal discharge or significant facial pain for at least three consecutive days at the start of illness. This pattern points to a bacterial cause from the beginning rather than a cold that turned into something worse.
- Double sickening: You start to feel better after a few days, then suddenly get worse again around day 5 or 6, with a new fever, worsening headache, or increased nasal discharge. This rebound pattern is a strong signal that a secondary bacterial infection has developed on top of the original viral illness.
If your symptoms don’t fit any of these three patterns, you almost certainly have a viral infection. Antibiotics won’t help, and your doctor will likely recommend managing symptoms at home while your body clears it.
The Physical Exam
During an office visit, your doctor will look inside your nose using a lighted instrument to check for swelling, redness, and the color of your nasal discharge. Thick, yellow-green discharge coming from the sinus drainage areas is a visual clue, though discharge color alone isn’t reliable enough to confirm a bacterial infection. They’ll also press on your cheeks and forehead to check for tenderness over the sinus cavities, and feel your neck for swollen lymph nodes, which can point toward an infectious cause.
This basic exam, combined with the symptom timeline, is enough for most diagnoses. The majority of people who visit a doctor for sinus symptoms will leave with a diagnosis based entirely on this clinical assessment.
Nasal Endoscopy
If your infections keep coming back, don’t respond to treatment, or your doctor suspects something structural is going on, they may use a nasal endoscope. This is a thin, flexible tube with a tiny camera and light on the end. After numbing the inside of your nose with a spray, the doctor threads the scope through your nasal passages while you sit upright in an exam chair.
The camera gives a magnified view of areas that can’t be seen during a standard exam. Your doctor can check for polyps (small growths that block drainage), unusual swelling patterns, or structural issues like a deviated septum. They can also collect pus directly from an infected sinus opening, which can then be sent to a lab to identify exactly which bacteria are causing the problem. This culture is one of the few ways to get a definitive bacterial diagnosis, and it’s especially useful when standard antibiotics haven’t worked.
The procedure takes only a few minutes and causes mild pressure but is generally not painful with the numbing spray.
When Imaging Comes In
CT scans are not routine for a typical sinus infection. They’re reserved for cases where symptoms are severe, recurrent, or not improving with treatment. A CT scan can reveal fluid levels in the sinuses, thickened sinus walls, and blockages that aren’t visible on a physical exam. It’s also the standard imaging choice before sinus surgery, giving surgeons a detailed map of the sinus anatomy.
Standard X-rays are rarely used anymore because they lack the detail to see the smaller sinus cavities clearly. CT scans have largely replaced them for any situation where imaging is genuinely needed.
One situation where imaging becomes urgent: if there are signs the infection may be spreading beyond the sinuses. A bulging eye, limited eye movement, changes in vision, or a severe headache with fever (particularly in teenagers and young adults) are red flags that warrant a CT scan with contrast right away. These complications are uncommon but serious, and a scan can reveal whether the infection has spread into the eye socket or toward the brain.
Ruling Out Allergies
Sinus pressure and congestion aren’t always from an infection. Allergic rhinitis causes many of the same symptoms, and distinguishing the two shapes what treatment you actually need. Several physical clues help your doctor sort this out. Dark circles under the eyes, itchy or watery eyes, wheezing, and eczema all point toward allergies rather than infection. Swollen lymph nodes and thick, discolored, or bloody discharge lean toward infection.
If the picture is unclear, especially with chronic or recurring symptoms, your doctor may recommend allergy testing. The most common version is a skin prick test, where tiny amounts of common allergens are placed on your forearm or back to see which ones trigger a reaction. A blood test measuring allergen-specific antibodies is an alternative when skin testing isn’t practical, such as when you’re taking antihistamines that would interfere with the results. These tests don’t diagnose a sinus infection directly, but they can explain why your sinuses are chronically inflamed and help prevent repeated misdiagnosis.
What Most People Can Expect
For the vast majority of sinus infections, the diagnostic process is straightforward: a conversation about your symptoms, a look inside your nose, and a clinical judgment call. No blood draws, no scans, no cultures. The whole process typically takes a single office visit. Endoscopy, imaging, and allergy testing enter the picture only when infections are unusually severe, keep returning, or resist initial treatment. If you’re on your first sinus infection of the season and your symptoms have lasted more than 10 days without improvement, that timeline is the single most important piece of diagnostic information you can bring to your appointment.