Most sinus infections are diagnosed in a regular office visit based on your symptoms and how long you’ve had them. Doctors rarely need imaging or lab tests. The key factor is the pattern of your illness: how long it’s lasted, whether it’s getting worse, and how severe your symptoms are. These details tell your doctor more than almost any test can.
The Symptom Timeline Matters Most
The single most important diagnostic tool for a sinus infection is the clock. A typical cold causes many of the same symptoms, including congestion, facial pressure, and nasal discharge. What separates a bacterial sinus infection from a viral one is how long those symptoms stick around and how they behave over time. Doctors look for one of three specific patterns.
The first is persistent symptoms. If you’ve had nasal discharge (any color), a daytime cough, or both for more than 10 days with no improvement, that timeline alone points toward a bacterial infection. A regular cold typically starts improving within 7 to 10 days.
The second is what’s sometimes called “double sickening.” You start feeling better after the first 5 to 6 days of a cold, then suddenly get worse again. New fevers, worsening headache, or an increase in nasal discharge after that initial improvement suggest bacteria have moved in on top of the original viral infection.
The third is a severe onset. If you develop a fever of 102.2°F (39°C) or higher along with thick, discolored nasal discharge that lasts at least 3 consecutive days from the start, that combination points to a bacterial cause right away, without needing to wait 10 days.
If your symptoms have been improving at any point during the illness and continue on that trajectory, your doctor will generally conclude it’s viral and antibiotics aren’t needed.
What Happens During the Physical Exam
Your doctor will look inside your nose with a lighted instrument to check for swelling, redness, and the type of discharge present. They may press on your cheeks and forehead to check for tenderness over the maxillary and frontal sinuses. A combination of specific findings makes the diagnosis more confident: pain in your upper teeth, visible pus in your nasal passages, colored nasal discharge you’ve noticed at home, poor response to over-the-counter decongestants, and abnormal results from transillumination (shining a light through the sinus to see if it’s blocked). Having four or more of these findings together makes a sinus infection much more likely, while having none of them makes it unlikely.
Transillumination involves placing a bright light against your cheek or forehead in a darkened room. A healthy, air-filled sinus transmits light well. A sinus filled with fluid or thickened tissue blocks it. This is a quick, painless check, though it’s less commonly used today than it once was.
Nasal Endoscopy for Complicated Cases
If your sinus infections keep coming back or your doctor suspects something beyond a straightforward infection, they may use nasal endoscopy. This involves threading a thin, flexible tube with a tiny camera and light into your nose. It gives a direct view of your nasal passages and the small openings where your sinuses drain. Your doctor can see swelling, polyps, structural problems, or pus draining from a specific sinus opening. The procedure takes only a few minutes and is done in the office, usually with a numbing spray applied to your nose beforehand.
When Imaging Comes Into Play
For a straightforward sinus infection, imaging isn’t necessary. A CT scan won’t help your doctor manage a single episode of acute sinusitis any better than a clinical exam will. Guidelines specifically recommend against routine imaging for uncomplicated cases.
CT scans become useful in specific situations: sinus infections that keep recurring, chronic sinusitis lasting 12 weeks or longer, symptoms that don’t respond to treatment, or when surgery is being considered. A CT scan maps the anatomy of your sinuses in detail and can reveal fluid levels, complete blockage of a sinus cavity, or structural issues like a deviated septum or polyps. That said, CT findings can be nonspecific. Sinus inflammation shows up on scans even during an ordinary cold, so a scan alone doesn’t confirm a bacterial infection. The more distinctive findings, like visible fluid levels or a completely opaque sinus, only appear in about 60% of confirmed cases.
Plain X-rays of the sinuses are rarely used anymore because CT scans provide far more detail when imaging is actually warranted.
Sinus Cultures: Reserved for Tough Cases
Doctors almost never culture a sinus infection on the first round. Cultures become relevant when treatment fails, when infections keep coming back, or when your doctor needs to identify the exact bacteria involved to choose the right antibiotic.
The most reliable way to get a culture is through maxillary sinus puncture, where a needle is inserted through the wall beneath your cheek to draw fluid directly from the sinus cavity. This sounds dramatic, and it is an uncommon procedure, typically performed by an ear, nose, and throat specialist. The sample is sent for a gram stain to look for white blood cells and bacteria, plus a full culture to identify the specific organism and test which antibiotics will work against it. Cultures can also be collected during nasal endoscopy by sampling discharge at the sinus drainage openings, though this method is considered slightly less precise than a direct puncture.
Ruling Out Conditions That Mimic Sinusitis
Facial pressure and congestion aren’t always a sinus infection. Allergic rhinitis is one of the most common mimics. If your doctor suspects allergies are driving your symptoms, they may order allergy testing through skin prick tests or a blood test that measures your immune response to specific allergens like dust, mold, or pollen. Allergies tend to cause clear, watery discharge, sneezing, and itchy eyes, while bacterial sinusitis produces thicker, discolored discharge with facial pain and sometimes fever. In some cases, a nasal swab can be examined under a microscope to look at the types of immune cells present, which helps distinguish allergic inflammation from an infection.
Migraines and tension headaches also frequently get mistaken for sinus infections. If you have recurring “sinus headaches” that never produce thick discharge or fever, your doctor may explore whether a headache disorder is the real cause. The overlap is common enough that studies have found a significant percentage of people who believe they have chronic sinus problems actually have migraines.