There is no single definitive test for a sinus infection. Most sinus infections are diagnosed based on your symptoms and how long they’ve lasted, not a lab test or scan. Doctors rely on a combination of what you describe and what they can see during a physical exam to determine whether your sinuses are infected and whether the cause is viral or bacterial.
How Most Sinus Infections Are Diagnosed
For a straightforward sinus infection, your doctor will ask about your symptoms and their timeline. The key symptoms are nasal congestion, thick or discolored nasal discharge, facial pain or pressure, and reduced sense of smell. The critical factor is duration: a viral sinus infection typically starts improving after five to seven days, while a bacterial infection often persists for seven to ten days or longer, and may actually worsen after day seven.
This worsening pattern, sometimes called “double sickening,” is one of the most reliable clues that a bacterial infection has developed. You start to feel better, then your symptoms return or get noticeably worse. That shift in trajectory matters more to your doctor than any single symptom on its own.
During a physical exam, your doctor may press on the areas over your sinuses to check for tenderness, particularly above the inner corner of the eye (over the frontal sinus) or along the cheeks. They might also shine a light through your sinuses, a technique called transillumination, to look for fluid buildup. However, both of these exam techniques have limited accuracy and are not reliable enough on their own to confirm or rule out an infection.
When Imaging Comes Into Play
CT scans and other imaging are not recommended for a typical acute sinus infection. According to American College of Radiology guidelines, CT imaging is unnecessary for patients with a straightforward clinical diagnosis, partly because scans cannot reliably distinguish a bacterial sinus infection from a viral one.
Imaging becomes appropriate in specific situations. If your doctor suspects a complication, such as the infection spreading toward your eye socket or brain, a CT scan with contrast dye is the standard next step. It can identify fluid collections, abscesses, and bone erosion with high accuracy. For people with chronic sinusitis (symptoms lasting 12 weeks or more) or recurrent infections who may be candidates for surgery, a CT scan without contrast is useful for mapping sinus anatomy and planning the procedure. And if your doctor suspects a fungal sinus infection or a mass, CT can reveal characteristic patterns like unusually dense material inside the sinuses or bone destruction.
MRI is reserved for more complex scenarios. It’s typically added when extensive complications are suspected, particularly those involving the brain or multiple areas around the sinuses. MRI is better at distinguishing between different types of soft tissue, which helps when the picture is unclear on CT alone.
Nasal Endoscopy
If a standard look inside your nose doesn’t explain your symptoms, or if your doctor suspects chronic sinusitis, polyps, or something unusual, they may use a nasal endoscope. This is a thin, flexible tube with a camera that gives a much more detailed view of your nasal passages and sinus openings. It can reveal pus draining from a specific sinus, swollen tissue, polyps, or other abnormalities that a basic exam would miss.
For chronic sinusitis specifically, current clinical guidelines recommend that doctors objectively confirm inflammation in the sinuses rather than relying on symptoms alone. Endoscopy is one of the primary ways to do this, alongside CT imaging.
Sinus Cultures
You might wonder whether a swab or culture can identify the exact bacteria causing your infection. The gold standard for this is a procedure called antral lavage, where a needle is used to puncture the sinus cavity beneath the eye and withdraw fluid for culture. This can definitively identify whether bacteria are present and which type. But as researchers at the University of Georgia have noted, it’s impractical for routine use, and it’s rarely performed outside of research settings or highly complicated cases.
Simpler nasal swabs are sometimes taken, but they sample the nasal passage rather than the sinus cavity itself. This means they can pick up bacteria that are simply living in your nose without actually causing the sinus infection, making them less reliable. Cultures are most useful when an infection hasn’t responded to treatment and your doctor needs to identify resistant bacteria or an unusual organism.
Blood Tests and Newer Approaches
Standard blood tests like white blood cell count are not routinely used to diagnose sinus infections. One marker that has shown some promise is C-reactive protein (CRP), a general indicator of inflammation in the body. Research has found that a CRP level below 10 mg/L provides some evidence against a bacterial sinus infection, while a level above 20 mg/L leans toward supporting the diagnosis. These thresholds aren’t precise enough for a definitive answer, but studies in Europe have shown that when doctors can see CRP results suggesting a bacterial infection is unlikely, they prescribe fewer unnecessary antibiotics.
Point-of-care tests that could be done in a doctor’s office, combining CRP with other quick assessments, are an area of active interest but not yet part of standard practice.
Allergy Testing for Chronic Sinus Problems
If your sinus infections keep coming back or never fully clear up, your doctor may investigate whether allergies are driving the inflammation. This is especially relevant for a condition called allergic fungal sinusitis, where the immune system overreacts to fungal organisms in the sinuses. In these cases, blood tests measuring immune markers (specifically IgE antibodies) are typically elevated, and skin prick testing can identify which allergens are triggering the response. Skin testing tends to be more sensitive than blood-based allergy tests for detecting fungal sensitivities.
Allergy testing won’t diagnose an acute sinus infection, but it can explain why someone keeps getting them and guide longer-term treatment to reduce inflammation and prevent recurrences.
What This Means in Practice
If you’re dealing with a sinus infection right now, the reality is that your doctor will likely diagnose it based on your symptoms and how long you’ve been sick. No scan or blood draw is needed in most cases. The most useful thing you can track before your appointment is exactly when your symptoms started, whether they improved and then worsened, and whether you have fever, severe facial pain, or swelling around your eyes, all of which signal that something beyond a routine infection may be happening and could prompt your doctor to order imaging or further testing.