Glaucoma testing involves several different exams, not just one. Because glaucoma can damage your vision without any noticeable symptoms, doctors use a combination of pressure measurements, nerve imaging, drainage angle checks, corneal thickness readings, and peripheral vision mapping to catch it early and track it over time. A basic screening might only include one or two of these, but a comprehensive glaucoma workup uses most or all of them.
Eye Pressure Measurement (Tonometry)
The most familiar glaucoma test measures the pressure of fluid inside your eye, called intraocular pressure. Normal pressure falls between 10 and 21 mmHg. Readings above 21 mmHg raise concern, though high pressure alone doesn’t confirm glaucoma, and some people develop glaucoma with pressure that never exceeds the normal range.
There are two main ways doctors measure this. The “air puff” test, formally called non-contact tonometry, pushes a quick burst of air at your eye and measures how much your cornea changes shape in response. It’s fast, requires no contact with your eye, and is commonly used during routine screenings. The more precise version is applanation tonometry, where a tiny flat-tipped instrument gently touches the surface of your numbed eye and measures how much force it takes to slightly flatten the cornea. Applanation is considered the gold standard for accuracy and is typically used when an air puff test returns unusual or borderline readings.
Optic Nerve Examination
Glaucoma damages the optic nerve at the back of your eye, so examining that nerve directly is one of the most important parts of testing. For this, your doctor will use dilating drops to widen your pupils, then look at the nerve through a slit lamp (a specialized microscope with a bright light). The drops cause some stinging and take about 20 minutes to work. Your vision will be blurry for up to six hours afterward, and bright lights will feel uncomfortably intense during that time.
What your doctor is looking for is very specific. The optic nerve has a natural indentation in its center called the “cup,” and doctors measure how large that cup is relative to the overall disc. If the cup-to-disc ratio differs by 0.2 or more between your two eyes, that raises suspicion for glaucoma. They also look for thinning of the nerve tissue around the rim, small splinter-shaped hemorrhages near the disc (which can signal glaucomatous damage and often precede visible nerve loss by 15 to 18 months), and changes in how blood vessels sit on the nerve. Vessels that appear to bend sharply or drift away from the rim edge suggest the nerve tissue underneath has been lost.
Doctors often photograph the optic nerve at baseline so they can compare it to future images and detect subtle changes over years.
Visual Field Testing (Perimetry)
This test maps your peripheral vision to find blind spots you may not even notice. You sit in front of a bowl-shaped machine, stare at a fixed central point, and press a button every time you see a small light flash somewhere off to the side. The machine varies the brightness and location of the lights to measure how sensitive your vision is at each point across your field of view.
Glaucoma tends to cause vision loss in recognizable patterns, particularly affecting peripheral vision first. The results appear as a map showing areas of normal sensitivity alongside any spots where vision has weakened or dropped out entirely. These patterns help your doctor distinguish glaucoma from other conditions that affect vision. Serial testing over time is especially useful because it reveals whether your visual field is stable or gradually shrinking.
Drainage Angle Assessment (Gonioscopy)
Fluid inside your eye drains through a ring-shaped channel where the colored part of your eye (the iris) meets the white outer wall. If this drainage angle is too narrow or blocked, fluid backs up and pressure rises. Gonioscopy lets your doctor see this angle directly.
During the test, a special lens is placed on the surface of your numbed eye, allowing your doctor to look sideways into the drainage channel. This is how they determine whether you have open-angle glaucoma (where the angle looks normal but drainage is sluggish) or angle-closure glaucoma (where the angle is physically blocked). The distinction matters because the two types are managed differently.
Corneal Thickness Measurement (Pachymetry)
Your corneal thickness directly affects how accurate your pressure readings are, which is why this measurement matters for glaucoma testing. The average central cornea in the U.S. is between 540 and 550 micrometers thick. If your cornea is thicker than average, your pressure readings may appear artificially high, making it look like you have a problem when you don’t. If your cornea is thinner than average, your true pressure may be higher than the reading suggests, potentially masking a real issue.
This is particularly important for people who have had laser vision correction surgery, which thins the cornea permanently. Without accounting for that change, pressure readings can be misleadingly low.
Nerve Fiber Imaging (OCT)
Optical coherence tomography, or OCT, uses light waves to create a detailed cross-sectional image of the retinal nerve fiber layer at the back of your eye. It measures the thickness of this nerve tissue in micrometers, giving your doctor a precise, objective number to track over time. Even before you notice any vision changes, OCT can detect thinning of the nerve fibers that suggests early glaucoma.
This test is quick and painless. You look into a machine while it scans your eye, and the results are compared against a database of normal values for your age. Doctors consider you a glaucoma suspect if your cup-to-disc ratio is greater than 0.5 or if OCT shows generalized or focal nerve fiber thinning. Notably, people with smaller-than-average optic discs may have glaucomatous thinning that doesn’t get flagged by standard cup-to-disc measurements alone, making OCT especially valuable for catching cases that might otherwise be missed.
When Pressure Is Normal but Glaucoma Is Present
About one form of the disease, called normal-tension glaucoma, pressure stays below 21 mmHg yet the optic nerve still deteriorates. This makes it a diagnosis of exclusion: doctors have to rule out other conditions that can mimic it, including inflammatory diseases, blood flow problems in the carotid arteries, and even pressure fluctuations that spike during activities but return to normal by the time you’re sitting in an exam chair.
If your doctor suspects normal-tension glaucoma, the workup may expand to include 24-hour blood pressure monitoring, blood tests for inflammatory or infectious conditions, and sometimes brain imaging to rule out problems further along the visual pathway. This is especially likely if you’re younger than 65, if damage is progressing rapidly, or if one eye is significantly worse than the other.
Screening vs. Comprehensive Exam
A basic vision screening, the kind offered at a health fair or quick optometry visit, typically checks only your visual sharpness and may include an air puff pressure reading. That’s not enough to detect glaucoma reliably. A comprehensive dilated eye exam includes the full set of tests described above: pressure measurement, optic nerve evaluation through dilated pupils, visual field testing, and often OCT and gonioscopy. If you have risk factors like a family history of glaucoma, are over 60, or are of African or Hispanic descent, the comprehensive version is what you need.