How the GDx Eye Test for Glaucoma Works

The GDx is a test that has proved its usefulness in the diagnosis and management of glaucoma. The GDx uses a laser to determine the thickness of the nerve fiber layer in the back of the eye. Older glaucoma tests have centered around measuring eye pressure or measuring the effect that glaucoma has on your overall visual field. Although these tests are extremely important in the treatment and management of glaucoma, it is helpful to use tests like the GDx to detect changes in the nerve fiber layer before significant damage is done causing irreversible vision loss.

Close up of a person's blue eye
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How It Works

The nerve fiber layer consists of millions of individual fibers called "axons" that surround the optic nerve and spread out over your retina. In many patients with glaucoma, considerable nerve fiber layer damage may have already occurred by the time any vision loss is noticed. Glaucoma refers to a group of diseases that cause damage to the optic nerve. The optic nerve is responsible for carrying images to the brain. Glaucoma is known as the "sneak thief of sight," because many people are not aware they have a problem until significant vision has been lost.

The GDx uses a type of scanning laser polarimeter to measure the thickness of the nerve fiber layer. The procedure is painless and is usually performed on an undilated pupil. The thickness of the nerve fiber layer is then compared with the nerve fiber layer of normal eyes. The GDx maps the nerve fibers and compares them to a database of healthy, glaucoma-free patients. A thinning of the fibers indicates glaucoma. This information is then made available to your healthcare provider in the form of pictures, graphs and statistical data that indicate the probability of glaucoma.

The GDx test is particularly powerful in early detection as studies show that more than 50% of individuals with glaucoma aren't aware they have it. It is also very useful for managing glaucoma over the years because it detects very small changes when compared with previous GDx data. This information is helpful to your healthcare provider so that he or she can decide if you truly have glaucoma or should only be considered a "glaucoma suspect." While nerve fiber analysis as a standalone test does not make for a definitive diagnosis of glaucoma, it does provide your practitioner with data that will add to your overall case so that better decisions may be made regarding your treatment.

Although the GDx is still a good test, there is another instrument that has even greater diagnostic accuracy. The development of the GDx and other instruments lead to the production of optical coherence tomography (OCT).

What Is an OCT?

OCT is a noninvasive imaging technology used to obtain high-resolution cross-sectional images of the retina. OCT is similar to ultrasound testing, except that imaging is performed by measuring light rather than sound. OCT measures the retinal nerve fiber layer thickness in glaucoma and other diseases of the optic nerve.

OCT is a way for optometrists and ophthalmologists to image the back of the eye including the macula, optic nerve, retina, and choroid. During an eye examination, optometrists and ophthalmologists can view the back of the eye and its anatomy. However, sometimes healthcare providers need more detail or need to inspect detail right below the surface which is difficult to view with standard techniques. In some ways, the OCT can be compared to doing an "MRI of the retina." Some describe it as an optical ultrasound because it images reflections between tissues to provide healthcare providers with cross-sectional images. The detail that can be visualized with an OCT is at such high resolution that medical professionals are seeing things never been seen before in a living human eye.

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Fallon M, Valero O, Pazos M, Antón A. Diagnostic accuracy of imaging devices in glaucoma: A meta-analysisSurvey of Ophthalmology. 2017;62(4):446-461. doi:10.1016/j.survophthal.2017.01.001

  2. American Academy of Ophthalmology. Glaucoma: the sneak thief of sight.

  3. National Eye Institute. At a glance: Glaucoma.

  4. Ahmed S, Khan Z, Si F, et al. Summary of glaucoma diagnostic testing accuracy: An evidence-based meta-analysisJ Clin Med Res. 2016;8(9):641-649. doi:10.14740/jocmr2643w

  5. Kuo AN, Verkicharla PK, McNabb RP, et al. Posterior eye shape measurement with retinal OCT compared to MRIInvest Ophthalmol Vis Sci. 2016;57(9):OCT196. doi:10.1167/iovs.15-18886

Additional Reading

By Troy Bedinghaus, OD
Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association.