Mastoiditis is diagnosed through a combination of physical examination, imaging, and sometimes lab work. The most reliable starting point is a hands-on exam of the area behind the ear, where tenderness and swelling over the mastoid bone are the most consistent signs. From there, a CT scan of the temporal bone is the gold standard for confirming the diagnosis and checking for complications.
What Doctors Look for During the Exam
The physical exam is usually the first and most important step. A doctor will press on the mastoid process, the bony bump you can feel behind your ear, checking for tenderness and swelling. These two findings are the most reliable indicators of acute mastoiditis. The skin over that area is often red and warm to the touch, and there may be visible swelling that pushes the outer ear forward and outward.
In more advanced cases, a pocket of pus can form between the bone and the tissue covering it (called a subperiosteal abscess). When this happens, the ear gets pushed noticeably to the side, and the natural crease behind the ear disappears entirely. Doctors will compare both sides of your head, since even subtle thickening of the tissue on one side can be a clue.
In adults, the most common symptoms that prompt evaluation are ear pain, ear drainage, and some degree of hearing loss. Fever is present in roughly 44% of mastoiditis cases. That’s a useful distinction from a straightforward ear infection, where fever shows up only about 32% of the time. Mastoid tenderness behind the ear occurs in about 65% of mastoiditis cases but is uncommon in uncomplicated ear infections, making it a key differentiator.
How Mastoiditis Differs From a Regular Ear Infection
This is the central diagnostic challenge. Nearly every case of mastoiditis starts as a middle ear infection (acute otitis media), and the two conditions overlap considerably. Both cause ear pain, and fluid behind the eardrum can be present in either one. The distinguishing features are what’s happening behind the ear: redness, swelling, and tenderness over the mastoid bone point toward mastoiditis rather than a simple infection that hasn’t spread.
A regular ear infection rarely produces retroauricular symptoms, the visible and palpable changes behind the ear. When those signs appear alongside ear pain, drainage, or hearing changes, the clinical picture shifts toward mastoiditis and typically triggers imaging to confirm.
CT Scans: The Primary Imaging Tool
A CT scan of the temporal bone is the imaging method of choice. It provides high-resolution views of the mastoid air cells, the small honeycomb-like pockets inside the mastoid bone. In mastoiditis, these air cells become clouded with fluid and infected material. The scan can also reveal something more concerning: erosion of the bony walls that separate these air cells, a hallmark of coalescent mastoiditis where the infection has begun destroying bone.
Specific findings doctors look for on CT include opacification (clouding) of the mastoid air cells, breakdown of the bony partitions between them, and erosion of key structures like the lateral wall of the mastoid or the thin bone separating the ear from the brain (the tegmen tympani). Erosion of these structures signals that the infection may be spreading beyond the mastoid into surrounding areas.
CT can be done with or without contrast dye. A scan without contrast is excellent for showing bone erosion and structural changes. Adding contrast makes it easier to identify soft tissue inflammation, fluid collections, and abscesses. According to the American College of Radiology, contrast-enhanced CT is particularly valuable when complications are suspected.
Plain X-rays, by comparison, are not sensitive enough to be useful. CT scans have largely replaced them for this diagnosis.
When MRI Enters the Picture
MRI is not a first-line tool for evaluating the mastoid bone itself. It doesn’t show bony detail as well as CT. However, MRI becomes the preferred choice when doctors suspect the infection has spread to nearby soft tissue or into the brain. It is the standard imaging method for detecting intracranial complications like brain abscesses or blood clot formation in the venous sinuses near the ear.
Venous sinus thrombosis, a clot that forms in the large drainage veins near the mastoid, is one of the more dangerous complications of mastoiditis. Imaging is considered the definitive way to diagnose it. CT of the temporal bone can raise suspicion by showing bone erosion near the sinus wall, but MRI (often with a specialized venography sequence) gives the clearest picture of the clot itself.
Blood Tests and Lab Work
Blood tests play a supporting role. They can’t diagnose mastoiditis on their own, but they help gauge the severity of infection and monitor for complications. C-reactive protein (CRP), a marker of inflammation, is commonly measured. In children, a CRP level above roughly 7.2 mg/dL has been suggested as a useful threshold for watching more closely for complications. White blood cell counts and other inflammatory markers may also be checked, though no single lab value confirms or rules out the diagnosis.
Fluid Cultures and Tympanocentesis
In some cases, doctors need to identify the specific bacteria causing the infection. This is done through tympanocentesis, a procedure where a small needle is used to draw fluid from behind the eardrum. The fluid is then sent to a lab to identify the pathogen and determine which antibiotics will work against it.
Tympanocentesis isn’t routine for every case. It’s reserved for situations where the infection hasn’t responded to antibiotics, where the patient is immunocompromised, or where mastoiditis is accompanied by serious complications like meningitis. In newborns or very young children who’ve spent time in intensive care, it may also be used early on because these patients are more likely to harbor unusual or resistant bacteria.
Masked Mastoiditis: A Harder Diagnosis
One particularly tricky scenario is “masked” or “latent” mastoiditis. This happens when someone has taken antibiotics for an ear infection, enough to partially suppress symptoms but not enough to fully clear the infection from the mastoid bone. The classic signs of redness, swelling, and tenderness behind the ear may be muted or absent, even though bone destruction is quietly progressing.
Masked mastoiditis is diagnosed largely through imaging. If someone has persistent or worsening ear symptoms despite antibiotic treatment, or develops vague headaches and low-grade malaise weeks after an ear infection, a CT scan can reveal the hidden damage. This is one reason doctors take ongoing ear symptoms seriously even when the outward signs seem to be improving.
Specialist Involvement
Mastoiditis typically triggers a referral to an ear, nose, and throat (ENT) specialist. For acute mastoiditis, an urgent consultation is recommended to evaluate whether the infection needs to be surgically drained. Chronic mastoiditis, where the infection has been smoldering for weeks or months, is managed primarily through surgery, and an ENT surgeon is central to that process. The decision about whether and when to operate depends on what imaging reveals, how the patient is responding to antibiotics, and whether complications have developed.