Sinus infections are diagnosed primarily through your symptoms and how long they’ve lasted, not through a specific lab test. Most of the time, a doctor can determine whether you have a sinus infection during a standard office visit without any imaging or bloodwork. Testing beyond a clinical evaluation is reserved for infections that keep coming back, don’t respond to treatment, or raise concern about complications.
Symptom Patterns Are the Primary Test
The most important diagnostic tool for a sinus infection is the timeline and combination of your symptoms. Doctors use three specific criteria, and meeting any one of them points toward a bacterial sinus infection rather than a regular cold:
- Symptoms lasting more than 10 days without meaningful improvement
- High fever (over 102°F) with thick, discolored nasal discharge or facial pain lasting 3 to 4 consecutive days at the start of illness
- “Double worsening” where symptoms start to improve, then get noticeably worse again within the first 10 days
That third pattern is particularly useful. A typical cold improves steadily after five to seven days. If you feel like you’re getting better and then take a sharp turn for the worse, that rebound suggests bacteria have taken hold in your already-inflamed sinuses. A bacterial sinus infection often persists for seven to ten days or longer and may actually worsen after the one-week mark, while a viral infection follows a more predictable arc of gradual improvement.
What Happens During the Physical Exam
If your symptom timeline raises suspicion, your doctor will do a focused exam. Using a nasal speculum (a small device that gently widens the nostril) and a directed light, they’ll look inside your nose for thick, discolored mucus draining from the sinus openings in the middle of your nasal passages. The color and consistency of that discharge matters: clear and watery points more toward allergies or a virus, while thick and yellow-green suggests bacterial infection.
Your doctor will also press on your cheeks, forehead, and the area between your eyes to check for tenderness. That said, sinus palpation has limited predictive value on its own. Tenderness can show up with a bad cold, and some people with confirmed bacterial infections don’t have much facial tenderness at all. It’s one piece of the puzzle, not a definitive answer.
How Doctors Rule Out Allergies
Sinus infections and allergic rhinitis share some overlapping symptoms, especially nasal congestion and drainage. But the pattern of symptoms helps distinguish them. Allergies cause watery (not thick or discolored) discharge, prominent itching in the nose and eyes, and frequent sneezing. They typically start within minutes of allergen exposure. Sinus infections, by contrast, produce facial pain or pressure, pain in the upper teeth, headaches, and reduced sense of smell. None of those are typical allergy symptoms. If your doctor suspects allergies are the real cause, or a contributing factor, allergy testing may be recommended instead of sinus-focused workup.
When a Specialist Uses an Endoscope
If your symptoms persist beyond 12 weeks or keep returning, you may be referred to an ear, nose, and throat specialist for nasal endoscopy. This involves threading a thin, flexible tube with a camera on the end into your nasal passages. It takes just a few minutes, and the area is typically numbed with a topical spray beforehand.
The specialist examines several key areas: the frontal recess (behind your forehead), the middle meatus (the main drainage pathway for your sinuses), and the space near the back of your nasal cavity. They’re looking for polyps, swelling of the tissue, thick purulent discharge, and any scarring from previous infections or surgeries. These findings are scored on a standardized scale that rates the severity of polyps, discharge, and swelling from absent to severe. Endoscopy provides a much more detailed view than a standard office exam and is especially important for diagnosing chronic sinusitis, where symptoms have been present for three months or more.
For a chronic sinusitis diagnosis, you need at least two of the following persisting for more than three months: thick discolored discharge, nasal congestion, facial pain or pressure, and reduced sense of smell. Ideally, there should also be visible confirmation of inflammation through endoscopy or imaging.
CT Scans and When They’re Needed
A straightforward sinus infection does not require imaging. CT scans enter the picture when infections are recurrent, don’t respond to treatment, or when surgery is being considered to map out the anatomy of your sinuses beforehand. They’re also ordered urgently if there’s concern about complications like the infection spreading toward the eye socket.
One important caveat about CT scans: they frequently show abnormalities that don’t actually mean you have a sinus infection. Up to 40 percent of adults with no sinus symptoms at all have mucosal thickening or other irregularities on sinus CT scans. Among people with minor colds, that number jumps above 80 percent. This is why CT findings are always interpreted alongside your symptoms and endoscopic exam. A scan showing sinus swelling in someone who just had a cold is meaningless on its own.
A standard non-contrast CT scan is sufficient for most situations. Contrast dye is added only when a complicated infection is suspected, such as an abscess forming near the eye.
Sinus Cultures Are Rare but Exist
The most definitive way to identify the exact bacteria causing a sinus infection is to collect fluid directly from the sinus cavity and grow it in a lab. This is done by inserting a thin catheter through the nasal wall into the maxillary sinus (behind your cheekbone) and aspirating a sample. It is not a routine procedure. Sinus cultures are typically reserved for patients with chronic sinusitis who have failed extensive treatment, including multiple rounds of antibiotics, steroid sprays, and saline rinses, and who are being prepared for surgery.
The culture identifies specific aerobic bacteria, anaerobic bacteria, and fungi, which helps guide targeted antibiotic choices when standard treatments haven’t worked. For the vast majority of sinus infections, this level of testing is unnecessary because the infection resolves with standard care or on its own.
What Most People Can Expect
If you go to your doctor with sinus symptoms, the visit will likely involve a conversation about how long you’ve been sick and how your symptoms have progressed, a look inside your nose, and some gentle pressing on your face. That’s usually enough to make the diagnosis. No blood test, swab, or scan is needed for a typical case. The key information your doctor needs from you is timing: when symptoms started, whether they’ve been getting better or worse, and whether you’ve experienced that telltale double-worsening pattern. Coming prepared with that timeline makes the diagnosis faster and more accurate.