Early detection and treatment of glaucoma are critical for preserving your vision, but diagnosing glaucoma can be tricky. Before making a diagnosis, your ophthalmologist will likely perform a comprehensive glaucoma exam, which consists of five tests:

• Tonometry  
• Ophthalmoscopy 
• Perimetry  
• Gonioscopy  
• Pachymetry  

Taken together, the data provided by these tests enable your doctor to make an informed diagnosis and design a course of treatment for your unique situation. Here, the Glaucoma Research Foundation explains what each of the diagnostic tests for glaucoma does and how each is performed.*

Tonometry

Tonometry measures your internal eye pressure, or intraocular pressure (IOP). During a tonometry test:

• Eye drops are used to numb the eye.
• Light pressure is applied to the eye with a puff of air to measure your eye’s resistance to the air hitting it.
• A small device, called a tonometer, is gently touched to the eye’s surface to measure the amount of pressure necessary to flatten the cornea (the clear front part of your eye).

Most tonometers are calibrated to measure IOP in millimeters of mercury (“mm Hg”). In most cases, the IOP of people with glaucoma will measure more than 20 mm Hg. Tonometry is one of the two tests used during a regular glaucoma checkup.

Ophthalmoscopy

Ophthalmoscopy measures the shape and color of your optic nerve. During an ophthalmoscopy exam:

• Eye drops are used to dilate (enlarge) your pupil.
• The doctor uses a small device with a light on it to examine the optic nerve.

Ophthalmoscopy is the second test used during a regular glaucoma checkup. If your IOP is not within the normal range and your optic nerve looks abnormal, your doctor will likely recommend the next two tests: perimetry and gonioscopy.

Perimetry

Perimetry tests your complete field of vision—both central and peripheral (side)—for blind spots and measures the dimmest light you can see at each spot tested. During a perimetry test:

• You are asked to look straight ahead. 
• A light is shone at different spots in your peripheral (side) field of vision.
• You press a button every time you see the light click on.

There is a certain natural blind spot in our visual field, so you may experience a delay in seeing the light as it moves around that particular spot. This is normal and does not necessarily indicate visual damage. Your doctor may recommend taking the test more than once to compare the results.

Gonioscopy

Gonioscopy determines whether the angle where your iris meets the back of your cornea is wide or narrow. During a gonioscopy exam:

• Eye drops are used to numb your eye.
• A hand-held contact lens containing a mirror is placed on your eye.

Both wide and narrow angles are potential indications of glaucoma.

Pachymetry

Pachymetry uses an ultrasonic wave instrument to measure the thickness of your cornea. This test is painless and simple to perform, and both eyes can be measured in about a minute.

During a pachymetry test:

• Eye drops are used to numb the eye.
• The doctor places a small probe, called a pachymeter, against the surface of your eye.

Though it’s a simple procedure, pachymetry is significant. The thickness of your cornea can affect your eye pressure readings. Your corneal thickness readings help the doctor interpret your IOP readings more accurately.

Optical Coherence Tomography

Additional tests are optical coherence tomography (OCT) and optical coherence tomography angiography (OCTA). These are non-invasive imaging tests that use light waves to take cross-sectional photos of your retina to diagnose and monitor glaucoma and other eye diseases.

How often should I get an eye exam?

It’s important to keep your annual eye exams, especially if you have any of the high-risk factors for glaucoma, including:

• Diabetes, 
• High blood pressure, and
• Family history of glaucoma.

Though people aged 60 and above are at greater risk, experts recommend getting a baseline glaucoma screening at age 40. Early signs of eye disease may begin around this time. Your eye doctor may also recommend more frequent eye exams based upon your screening results. 

*Glaucoma Research Foundation. (2025). Testing for Glaucoma. https://glaucoma.org/understanding-glaucoma/testing 

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