Glaucoma is detected through a series of eye tests, not just one. No single measurement can confirm or rule it out, so eye doctors use a combination of pressure checks, nerve imaging, and vision mapping to build a complete picture. Most of these tests are painless and take only a few minutes each.
Eye Pressure Measurement
The most familiar glaucoma test is tonometry, which measures the pressure of fluid inside your eye. Normal eye pressure falls between 10 and 20 millimeters of mercury (mmHg). Readings above that range don’t automatically mean you have glaucoma, but they do raise the level of concern. Elevated pressure with no other symptoms is called ocular hypertension, and it puts you at higher risk.
During the most common version of this test (Goldmann applanation tonometry), your eye doctor numbs your eye with drops and gently touches the surface with a small, flat-tipped instrument. You won’t feel pain, just light pressure for a second or two. Some offices use a device that puffs a quick burst of air at your eye instead, which requires no contact at all. Both approaches give a pressure reading in seconds.
One important caveat: your corneal thickness affects the accuracy of pressure readings. Thicker corneas can make pressure appear higher than it actually is, and thinner corneas can mask dangerously high pressure. The average corneal thickness is around 541 microns. To account for this, many doctors also perform pachymetry, a quick, painless measurement of your cornea’s thickness using an ultrasound probe or light-based scanner. This helps them interpret your pressure reading more accurately.
Optic Nerve Examination
Pressure numbers alone don’t tell the whole story. Some people develop glaucoma damage at pressures considered “normal,” while others tolerate higher pressures without harm. That’s why directly examining the optic nerve is one of the most important parts of a glaucoma evaluation.
Your doctor will dilate your pupils with eye drops (which takes about 20 to 30 minutes to take full effect) and then look at the back of your eye with a magnifying lens and a bright light. They’re examining the optic nerve head, a small disc where roughly one million nerve fibers exit the eye on their way to the brain. In the center of this disc is a natural depression called the “cup.” The ratio of the cup’s size to the overall disc size is a key indicator. When glaucoma damages nerve fibers, the cup gets larger relative to the disc. A large or enlarging cup is the second most significant risk factor for chronic open-angle glaucoma, after elevated eye pressure.
A large cup doesn’t automatically mean damage. Some people are simply born with bigger cups. What matters most is whether the cup is changing over time, which is why doctors take periodic photographs of the optic nerve to track the cup-to-disc ratio at each visit.
Visual Field Testing
Glaucoma typically steals peripheral vision first, often so gradually that you don’t notice until significant damage has occurred. Visual field testing (perimetry) maps your entire field of vision to catch blind spots you can’t detect on your own.
The most common version is automated static perimetry. You sit with your chin on a rest and look into a bowl-shaped machine. Tiny spots of light flash at different locations and brightnesses across your visual field, and you press a button each time you see one. The machine varies the brightness and position of the lights to measure how sensitive your vision is at each point. The whole process takes roughly five to ten minutes per eye.
There’s also kinetic perimetry, where the person administering the test moves an object around and you report when it appears. This version is less common for routine glaucoma screening but useful in certain situations.
The results produce a detailed map showing any areas where your vision is weaker than expected. Certain patterns of vision loss are characteristic of glaucoma and distinct from other eye diseases, which helps your doctor pinpoint the cause. A test showing visual field loss means vision in some areas isn’t as sensitive as it should be. The loss could be a small blind spot or, in advanced cases, large missing regions.
Drainage Angle Assessment
Fluid constantly flows in and out of your eye through a drainage channel located where your iris meets the white of your eye. If this channel is too narrow or blocked, fluid backs up and pressure rises. A test called gonioscopy lets your doctor see this drainage angle directly.
During gonioscopy, your eye is numbed and a special lens is placed gently on the surface. This lens contains a mirror that lets the doctor peer into the drainage angle, a space that’s otherwise hidden behind the curve of the cornea. The test takes just a minute or two and is painless.
Gonioscopy is the test that distinguishes between the two main types of glaucoma. If the angle is open but not draining properly, that’s open-angle glaucoma, the most common form. If the angle is physically narrow or closed, that’s angle-closure glaucoma, which can develop slowly or strike suddenly as an emergency. The distinction matters because the two types require different treatment approaches.
Nerve Fiber Imaging With OCT
Optical coherence tomography (OCT) is one of the most valuable tools for catching glaucoma early. It uses light waves to create a highly detailed cross-sectional image of your retina, measuring the thickness of the nerve fiber layer with precision down to 10 to 20 micrometers (roughly one-fifth the width of a human hair).
The test is completely non-contact. You sit in front of the machine and look at a target while it scans your eye in seconds. There’s no puff of air, no drops, no discomfort.
What makes OCT so useful is that it can detect thinning of the nerve fiber layer before you notice any vision changes and sometimes before visual field tests pick up a problem. The machine compares your measurements against a database of normal values for your age, flagging areas that fall outside the expected range. Over multiple visits, OCT can track whether your nerve fiber layer is stable or gradually thinning, giving your doctor an objective way to measure whether treatment is working.
How These Tests Work Together
No single test confirms glaucoma. Your doctor pieces together results from pressure readings, nerve appearance, visual field maps, drainage angle assessment, and nerve fiber thickness to make a diagnosis. Someone with borderline pressure but a healthy nerve and normal visual fields might be monitored closely rather than treated immediately. Someone with “normal” pressure but a thinning nerve fiber layer and early field loss could be diagnosed and treated right away.
This is also why glaucoma testing isn’t a one-time event. Because the disease progresses slowly and silently, repeated testing over months or years is often necessary to distinguish true glaucoma from normal variation. Your doctor may want to see a clear trend before starting treatment.
When and How Often to Get Tested
The American Academy of Ophthalmology recommends a baseline comprehensive eye exam at age 40 for everyone. After that, the frequency depends on your age and risk level. For people without risk factors: every two to four years from ages 40 to 54, every one to three years from 55 to 64, and every one to two years after 65. If you have risk factors like a family history of glaucoma, African or Hispanic ancestry, high eye pressure, or very thin corneas, your doctor will likely recommend more frequent testing starting earlier.
Home Monitoring Devices
Home tonometers now exist for people already diagnosed with glaucoma who need to track their pressure between office visits. The iCare HOME device, for example, lets you measure your own eye pressure multiple times a day. In a clinical study, the device detected a meaningful treatment response (a 20% or greater pressure drop) in about 91% of the eyes where office-based testing also confirmed a response.
Home monitoring is particularly useful because eye pressure fluctuates throughout the day. A single office reading captures only one snapshot, while home measurements across different times can reveal patterns your doctor would otherwise miss. That said, these devices are prescribed and fitted by an eye care provider. They supplement professional exams rather than replace them, and they’re currently used for monitoring rather than initial diagnosis.