Glaucoma testing involves a series of painless eye exams that measure pressure inside the eye, check for optic nerve damage, and map your peripheral vision. No single test can diagnose glaucoma on its own. Instead, eye doctors combine results from several tests to determine whether the optic nerve is being damaged and whether treatment is needed.
Eye Pressure Measurement (Tonometry)
The most familiar glaucoma test measures intraocular pressure, or IOP. Normal eye pressure falls between 10 and 21 millimeters of mercury (mmHg). Pressure above 21 mmHg raises suspicion, but high pressure alone doesn’t confirm glaucoma. Some people develop optic nerve damage at lower pressures, and others tolerate higher pressures without any damage at all.
The gold standard is Goldmann applanation tonometry, where a tiny probe gently touches the surface of your numbed eye. You won’t feel pain because the doctor applies anesthetic drops first. Another common method is the “air puff” test (non-contact tonometry), which uses a quick burst of air directed at your eye. It’s startling for a second but completely painless. Portable devices like the iCare rebound tonometer use a lightweight probe that taps the cornea so briefly most people barely notice it. Your doctor may use whichever instrument is available, but the readings are interpreted the same way.
Corneal Thickness Measurement (Pachymetry)
Your corneal thickness directly affects how accurate your pressure reading is. Standard tonometry assumes a corneal thickness of about 500 micrometers. If your corneas are thinner than average, pressure readings come in artificially low, potentially masking a problem. Thicker corneas push readings higher, sometimes triggering unnecessary concern. Research shows that for every 10 micrometers of deviation from the standard thickness, pressure readings shift by roughly 0.1 to 0.3 mmHg depending on the instrument used.
Pachymetry takes only a few seconds per eye. A small ultrasound probe touches the surface of your numbed cornea and instantly measures its thickness. This is typically done once as a baseline so your doctor can factor the result into every future pressure reading.
Optic Nerve Examination
Looking directly at your optic nerve is one of the most important parts of a glaucoma evaluation. Your doctor will use dilating drops to widen your pupils, then examine the back of each eye with a bright light and magnifying lens. The key feature they’re looking for is the “cup-to-disc ratio,” which describes the size of the central depression (the cup) relative to the entire optic nerve head (the disc).
In healthy eyes, this ratio is typically around 0.3 to 0.4. In glaucoma patients, it averages around 0.75, reflecting the loss of nerve tissue that makes the cup appear larger. Your doctor also looks for asymmetry between your two eyes, since a significant difference in cup size can be an early warning sign even when both ratios are technically in range.
OCT Imaging
Optical coherence tomography, or OCT, uses light waves to create a detailed cross-sectional image of the retinal nerve fiber layer surrounding the optic nerve. This scan is completely non-contact. You simply rest your chin on a support and look at a target while the machine captures images in a few seconds.
The test measures the thickness of the nerve fiber layer in micrometers. In healthy eyes, the global average thickness runs about 112 micrometers, while glaucoma patients average closer to 89 micrometers. The scan breaks results down by quadrant, with the top and bottom of the optic nerve normally having the thickest fibers (around 137 to 140 micrometers in healthy eyes). Thinning in these areas often shows up before you notice any vision changes, making OCT one of the best tools for catching glaucoma early. Your results are color-coded green, yellow, or red based on comparison to a database of people your age.
Visual Field Testing (Perimetry)
Glaucoma typically damages side vision first, so a visual field test maps your peripheral vision in detail. You sit in front of a bowl-shaped instrument with one eye covered. Small lights of varying brightness flash at different locations inside the bowl, and you press a button each time you see one. The machine deliberately shows some lights too dim for anyone to see, which helps it find the exact threshold of your sensitivity at each point.
The test takes about five to ten minutes per eye and requires your concentration. It can feel tedious, and most people miss some lights simply from fatigue or momentary inattention. The software accounts for this. What your doctor looks for are consistent patterns of loss, particularly areas of reduced sensitivity in the upper or lower half of your visual field that match the anatomy of glaucoma damage. A result called the Glaucoma Hemifield Test flags whether the pattern falls outside normal limits. Meaningful results require at least two consistent tests, since a single abnormal field could reflect a bad day rather than true damage.
Drainage Angle Assessment (Gonioscopy)
Gonioscopy tells your doctor which type of glaucoma you may have. The fluid inside your eye drains through a tiny gap where the iris meets the cornea, called the drainage angle. In open-angle glaucoma (the most common type), this angle looks structurally normal but doesn’t drain efficiently. In angle-closure glaucoma, the iris physically blocks the drainage pathway.
To perform this test, your doctor places a special lens directly on the surface of your numbed eye. Through the lens, they can see the internal drainage structures and grade the angle on a scale from 0 (completely closed) to 4 (wide open). If the filtering tissue called the trabecular meshwork can’t be seen in more than half of the angle, you’re at high risk for angle closure. This distinction matters because the two types of glaucoma require different treatment approaches.
How Diagnosis Is Made
No single test result triggers a glaucoma diagnosis. Instead, the doctor looks for a consistent picture across multiple findings: elevated or suspicious pressure, thinning of the nerve fiber layer on OCT, a larger-than-expected cup-to-disc ratio, and characteristic visual field loss. Some people have elevated pressure but no nerve damage, a condition called ocular hypertension that requires monitoring rather than treatment. Others have normal pressure but clear signs of nerve damage, known as normal-tension glaucoma. The combination of structural damage to the optic nerve and functional vision loss on field testing is what confirms a diagnosis.
Because glaucoma causes irreversible damage and progresses without symptoms in its early stages, routine screening matters. The American Academy of Ophthalmology recommends a baseline comprehensive eye exam at age 40 for everyone. After that, people aged 40 to 54 with no risk factors should be evaluated every two to four years, those 55 to 64 every one to three years, and those 65 and older every one to two years. If you’re at higher risk due to family history, African American heritage, or other factors, more frequent testing is recommended starting before age 40.
What the Exam Feels Like
Most glaucoma tests are painless and quick. The main inconvenience comes from the dilating drops used to examine your optic nerve and perform OCT. Dilation typically lasts four to six hours, though it can persist up to 24 hours in people with lighter eye colors. During that time, your vision will be blurry for close-up tasks and your eyes will be sensitive to bright light. Bringing sunglasses to your appointment helps. You can drive home in most cases, but some people prefer to have someone else drive until the blur fades.
The full battery of tests, including pressure measurement, pachymetry, gonioscopy, OCT, and visual field testing, takes roughly 60 to 90 minutes. Not every visit requires every test. Initial evaluations tend to be the most thorough, while follow-up appointments focus on tracking changes in pressure, nerve fiber thickness, and visual field results over time.