Sinus infections are diagnosed primarily by symptoms and their timeline, not by lab tests or imaging. In most cases, a doctor can determine whether you have a sinus infection during a standard office visit by asking about what you’re feeling, how long it’s lasted, and whether symptoms have been getting better or worse. Scans and cultures are reserved for complicated or recurring cases.
The Symptoms Doctors Look For
The core diagnostic picture combines three things: thick or discolored nasal drainage, nasal congestion or obstruction, and facial pain or pressure. You don’t need all three, but you need at least two for a clinician to consider sinusitis. Loss of smell is another common marker. These symptoms overlap with a regular cold, which is exactly why timing matters so much in the diagnosis.
Your doctor will also check whether the discharge is coming from one side or both. One-sided drainage with facial pain on the same side points more strongly toward a bacterial infection. A fever above 100.4°F adds further weight to that diagnosis, especially when combined with worsening pain.
How Timing Separates Viral From Bacterial
The single most important diagnostic tool is a calendar. Most sinus infections start as viral infections (common colds), and the key question is how long your symptoms have lasted and what direction they’re heading.
If your symptoms have been present for fewer than 10 days and are gradually improving, you almost certainly have a viral sinus infection. No antibiotics needed, and no further workup required.
A bacterial sinus infection is diagnosed in two specific scenarios. First, if your symptoms persist for 10 days or longer without any improvement. Second, if you start getting better and then get noticeably worse again within 10 days. Doctors call this pattern “double worsening” or “double sickening,” and it’s one of the most reliable clinical signs that bacteria have taken over what started as a viral infection.
Sinus Infection vs. Allergies
Allergies and sinus infections can feel similar, but a few differences help doctors tell them apart. Allergies cause itching: itchy eyes, scratchy throat, and frequent sneezing. The nasal discharge with allergies is typically clear and watery. Sinus infections, by contrast, produce yellow or green discharge and are more likely to cause facial pressure, pain, and loss of smell. If you’re rubbing the corners of your eyes and sneezing constantly, that points toward allergies. If you’re dealing with thick discolored mucus and aching cheekbones, that points toward infection.
Physical Exam Techniques
During an office visit, your doctor will look inside your nose with a light, checking for swelling of the nasal lining, visible pus, and polyps. They’ll press on your cheeks and forehead to check for tenderness over the sinuses. An older technique called transillumination, where a light is held against the sinuses to see if they’re filled with fluid, is still occasionally used but isn’t very reliable. Studies show it matches imaging results only about 75% of the time, and it works poorly for infections affecting both sides. Most clinicians rely on symptom history over physical exam findings alone.
When Imaging Gets Involved
For a straightforward sinus infection, guidelines from the American Academy of Otolaryngology are clear: no imaging. A CT scan or X-ray adds nothing to the diagnosis of an uncomplicated case and can actually create confusion, since scans often show sinus inflammation during ordinary colds.
CT scans become appropriate in specific situations:
- Suspected complications. If you develop eye swelling, bulging of the eye, difficulty moving your eyes, or vision changes, a CT with contrast dye is used to check whether the infection has spread to the eye socket or brain.
- Chronic or recurring infections. If symptoms last 12 weeks or longer, or if you keep getting sinus infections several times a year, a CT without contrast helps identify structural problems, polyps, or other abnormalities that might explain why.
- Surgical planning. Before sinus surgery, CT imaging maps out the anatomy of your sinuses to reduce the risk of complications during the procedure.
- Immunocompromised patients. If you have a weakened immune system and develop sinus symptoms, doctors maintain a high level of suspicion for invasive fungal infections, which require imaging to catch early.
What an ENT Does Differently
If you’re referred to an ear, nose, and throat specialist, the main additional tool is nasal endoscopy. This involves sliding a thin, flexible tube with a camera into your nose to get a direct view of the sinus openings. The ENT is looking for pus draining from specific sinus passages, swollen tissue blocking drainage pathways, and polyps or other structural issues. The procedure takes a few minutes, involves some pressure but usually isn’t painful, and gives much more detail than a standard office exam with a light.
In some cases, the ENT may collect a sample of the drainage directly from the sinus opening during endoscopy. This culture can identify the exact bacteria involved, which is useful when infections haven’t responded to initial antibiotic treatment or keep coming back.
Acute vs. Chronic Classification
The duration of your symptoms determines how the infection is classified, which affects both the diagnostic approach and treatment plan. Acute sinusitis covers infections lasting up to four weeks. Subacute cases run from four to 12 weeks. Chronic rhinosinusitis is diagnosed when symptoms persist for 12 weeks or longer, and it typically requires endoscopy and CT imaging to confirm, because at that point the concern shifts from a simple infection to underlying causes like polyps, immune dysfunction, or anatomical problems that trap mucus.
Recurrent acute sinusitis, defined as four or more separate episodes per year with symptom-free intervals between them, also warrants a more thorough workup. The goal shifts from treating the current infection to figuring out why your sinuses keep getting infected in the first place.
Red Flags That Signal an Emergency
Most sinus infections resolve on their own or with routine treatment. But certain symptoms suggest the infection may be spreading beyond the sinuses, and these require immediate medical attention: pain, swelling, or redness around the eyes; double vision or other vision changes; high fever; confusion; and a stiff neck. These can indicate the infection has reached the eye socket or the lining of the brain, both of which need urgent treatment.