Most sinus infections are diagnosed based on your symptoms and how long they’ve lasted, not through any lab test or scan. A doctor will ask you specific questions about what you’re experiencing, look inside your nose, and use established timelines to determine whether your infection is viral or bacterial. Imaging and other tools are reserved for cases that don’t respond to treatment or that raise concern about complications.
The Symptom Timeline Matters Most
The single most important factor in diagnosing a sinus infection is how long your symptoms have been going on and whether they’re getting better or worse. The core symptoms doctors look for are thick, discolored nasal drainage combined with nasal congestion, facial pain or pressure, or both. But those symptoms alone don’t confirm a bacterial infection, because a common cold causes the same things.
Here’s how clinicians separate one from the other:
- Viral sinus infection: Symptoms have been present for fewer than 10 days and are not worsening. This is essentially a cold affecting the sinuses, and it resolves on its own.
- Bacterial sinus infection: Symptoms persist without any improvement for 10 days or longer, or they start to improve and then get noticeably worse again within 10 days. Doctors sometimes call that second pattern “double worsening.”
- Severe bacterial onset: A high fever above 102°F, significant one-sided facial pain, and purulent discharge with nasal obstruction lasting 3 or more days can point to a bacterial cause even before the 10-day mark.
That 10-day threshold is the cornerstone of diagnosis. Most colds clear up within 7 to 10 days. If yours plateaus or reverses course instead, your doctor has a strong basis for diagnosing a bacterial sinus infection.
What Happens During the Physical Exam
Your doctor will look inside your nose using a light and a speculum, a procedure called anterior rhinoscopy. They’re checking the color and condition of the tissue lining your nasal passages, looking for swelling, and noting whether there’s thick, pus-like drainage. They may also press on your cheeks, forehead, and the area between your eyes to check for tenderness over the sinuses.
Common findings during a sinus infection include pain over the cheeks that radiates to the forehead or teeth, pain that worsens when you bend forward, postnasal drip, a blocked nose, and reduced sense of smell. In some cases there may be redness or mild swelling of the skin over the cheeks, nose, or eyelids. No single finding on its own confirms a sinus infection. Doctors rely on the overall picture of your symptoms, their duration, and what they observe during the exam.
Symptoms That Raise or Lower the Odds
One finding that significantly increases the likelihood of a bacterial cause is cacosmia, a foul smell you detect in your own nose. Of all individual symptoms studied, this one raises the probability of bacterial sinusitis the most, pushing it to roughly 68%. Tooth pain and visible pus in the nasal discharge also make a bacterial infection more likely, though to a lesser degree.
One thing that does not help distinguish bacterial from viral: the color of your mucus. Despite the widespread belief that green or yellow discharge means you need antibiotics, the color of nasal drainage is unreliable for telling the two apart. Both viral and bacterial infections produce discolored mucus.
When an ENT Specialist Gets Involved
If your sinus infections keep recurring or don’t respond to treatment, you may be referred to an ear, nose, and throat specialist. An ENT can perform nasal endoscopy, which involves threading a thin, flexible tube with a camera through your nostril to get a detailed view of your sinus passages. This lets the doctor see exactly where pus is draining from, identify structural problems like a deviated septum or nasal polyps, and in some cases collect a sample of the discharge directly from the sinus opening for a culture. That culture can identify the specific bacteria involved and guide antibiotic choices.
Endoscopy is also a key part of diagnosing chronic sinusitis. Chronic sinusitis is defined by symptoms lasting 12 weeks or longer, and it requires objective evidence of inflammation, either through endoscopy or imaging, to confirm the diagnosis. Simply having congestion and facial pressure for months isn’t enough on its own.
When Imaging Is and Isn’t Needed
For a straightforward sinus infection, imaging is not recommended. The American College of Radiology states that scans are usually not appropriate for the initial evaluation of acute uncomplicated sinusitis lasting less than four weeks. A CT scan or X-ray won’t change how your doctor manages a typical case, and the sinuses of someone with a simple cold can look abnormal on imaging, leading to unnecessary treatment.
Imaging becomes important in specific situations:
- Suspected complications: If you develop swelling or redness around the eye, vision changes, or severe headache with a stiff neck, a CT scan (often with contrast dye) is used to check whether the infection has spread to the eye socket or brain.
- Surgical planning: If chronic sinusitis hasn’t responded to medical treatment and surgery is being considered, a CT scan without contrast is essential. It maps the bony anatomy of the sinuses and identifies structural variations that could affect the procedure.
- Suspected fungal infection: In patients with weakened immune systems, rapidly progressing symptoms can suggest an invasive fungal infection. Both CT and MRI play a role here, with MRI being particularly useful for evaluating whether the infection has reached the brain.
- Suspected mass or tumor: If symptoms are one-sided, include bloody discharge, or don’t fit the typical pattern, MRI with contrast is the preferred study.
Conditions That Mimic a Sinus Infection
Several conditions cause facial pressure, congestion, and headaches that feel exactly like a sinus infection but require different treatment. Allergic rhinitis is one of the most common mimics. It shares many symptoms with sinusitis but tends to involve itchy eyes and nose, sneezing in clusters, and clear (not thick) drainage. Allergies also follow seasonal or environmental patterns rather than the acute timeline of an infection.
Migraines are another frequent source of confusion. Facial pain and pressure from a migraine can center around the sinuses, and some migraines even cause nasal congestion. The difference is that migraines tend to be episodic, may come with light or sound sensitivity, and don’t produce purulent discharge. A simple cold also overlaps heavily with early sinusitis but is distinguished by the presence of body aches, general fatigue, and symptom resolution within 7 to 10 days.
Structural problems like a significantly deviated septum or nasal polyps can cause chronic one-sided obstruction that feels like a lingering infection. These are usually identified during a nasal exam or endoscopy and tend to produce congestion as the dominant symptom, with less runny drainage than an active infection would cause.
Red Flags That Need Immediate Attention
Sinus infections rarely become dangerous, but when they do, the infection has typically spread beyond the sinuses into nearby structures. Symptoms that signal this include swelling, pain, or redness around the eyes, double vision or other visual changes, a 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 require emergency evaluation with imaging and prompt treatment.
In young children, a sinus infection affecting the ethmoid sinuses (between the eyes) can cause noticeable swelling and redness of the skin around the eye. This is not uncommon in infants and toddlers and warrants urgent medical evaluation.