Most sinus infections are diagnosed based on your symptoms and how long they’ve lasted, not a specific lab test. Doctors rarely order imaging or cultures for a straightforward case. The real diagnostic challenge is figuring out whether your infection is viral (which will resolve on its own) or bacterial (which may need antibiotics), and that distinction comes down to a few key patterns in your symptoms.
How Doctors Diagnose a Sinus Infection
A sinus infection diagnosis starts with your symptom history. Clinicians look for a combination of major and minor symptoms. The major ones are purulent (thick, discolored) nasal discharge, nasal congestion, facial pain or pressure, facial fullness, reduced sense of smell, and fever. Minor symptoms include headache, ear pain or pressure, bad breath, dental pain, cough, and fatigue. A diagnosis typically requires either two major symptoms or one major symptom plus two or more minor ones.
During a physical exam, your doctor will press gently on the areas over your sinuses, specifically along the bony ridge below your eyebrows (frontal sinuses) and just beside your nose below the cheekbones (maxillary sinuses). Tenderness or pain during this palpation suggests inflammation in those sinus cavities. They’ll also look inside your nostrils for swelling, redness, and discharge.
Viral vs. Bacterial: The Critical Distinction
This is the part that matters most for your treatment. About 90% of sinus infections start as viral infections, essentially a cold that inflames the sinuses. Viral sinusitis doesn’t benefit from antibiotics. Bacterial sinusitis does, but it’s less common. Doctors use three timing patterns to tell them apart.
Persistent symptoms beyond 10 days: If your sinus symptoms haven’t improved at all after 10 days from when your cold started, that points toward a bacterial infection. A viral infection would normally be getting better by then.
Severe onset: A high fever (102°F or higher) combined with purulent nasal discharge or facial pain lasting at least 3 to 4 consecutive days right at the start of illness suggests bacteria are involved from early on.
Double sickening: You catch a cold, start feeling better after 5 to 6 days, then suddenly get worse again with new fever, increased nasal discharge, or headache. This rebound pattern is a strong indicator that a bacterial infection has set in on top of the original virus.
One symptom that’s particularly telling is cacosmia, a foul smell that you detect in your own nose. This finding raises the probability of bacterial sinusitis to roughly 68%. Tooth pain and thick, colored nasal discharge also increase the likelihood, though less dramatically.
When Imaging Is Used
If you’re expecting a scan to confirm your sinus infection, you probably won’t get one. Uncomplicated sinusitis does not require any imaging. A doctor can diagnose it from your symptoms alone, and a CT scan of someone with a common cold would often show sinus inflammation anyway, making it unreliable for distinguishing between a simple cold and a true sinus infection.
Imaging comes into play when your symptoms keep coming back, don’t respond to treatment, or suggest something more complicated. CT scans are the preferred choice over traditional X-rays because they provide far more detail, particularly for the smaller sinus cavities behind and between your eyes (the ethmoid and sphenoid sinuses) that plain X-rays visualize poorly. X-rays also can’t reliably distinguish between infection, polyps, and tumors when a sinus appears blocked. At many facilities, a limited CT scan costs about the same as a full set of X-rays while giving significantly more useful information.
CT scans also serve as a roadmap before sinus surgery, helping surgeons understand your unique sinus anatomy.
Nasal Endoscopy
For recurrent or chronic sinus problems, your doctor may use a nasal endoscope, a thin, flexible tube with a camera on the end. It’s guided through your nasal passages to give a direct view of the tissue inside. The camera can reveal swelling, polyps, the exact site of drainage or bleeding, and pus coming from a specific sinus opening. In some cases, the doctor will collect a pus sample directly from the infected area or take a small tissue biopsy during the procedure.
This isn’t a routine test for a single episode of sinusitis. It’s reserved for situations where the diagnosis is unclear, symptoms have persisted despite treatment, or your doctor needs to see exactly what’s happening inside before planning next steps.
Sinus Cultures and Lab Tests
Routine cultures are not recommended for a standard sinus infection. Most cases are managed in primary care without sending any samples to a lab, and acute sinusitis frequently resolves on its own.
Cultures become important in specific situations: when you’re at high risk for complications, when antibiotic resistance is suspected, or when previous rounds of treatment have failed. The most definitive method is sinus aspiration, where fluid is drawn directly from the sinus cavity with a needle. This identifies the exact bacteria causing the infection and which antibiotics will work against it. It’s an uncommon procedure reserved for complicated cases.
If a fungal sinus infection is suspected, particularly the invasive type that can affect people with weakened immune systems, a tissue biopsy may be taken and examined under a microscope to look for fungal elements before culture results come back.
What You Can Assess at Home
You can’t definitively diagnose a sinus infection at home, but you can track the patterns that help your doctor make the call. Pay attention to the color and thickness of your nasal discharge (thick yellow or green is more concerning than clear), whether you have facial pressure or pain concentrated over your cheeks or forehead, and how your symptoms change over time.
The most useful thing you can do is note your timeline. Write down when your symptoms started, whether they improved and then worsened again, and how many days you’ve had specific symptoms like fever or facial pain. This information maps directly onto the diagnostic criteria your doctor will use. A cold that’s been getting steadily worse for 12 days tells a very different story than one that peaked at day 3 and is slowly improving.