No single test can definitively diagnose glaucoma on its own. The most accurate approach combines two types of assessment: structural imaging of the optic nerve (typically with OCT) and functional testing of your visual field (with automated perimetry). The World Glaucoma Association identifies progressive thinning of the retinal nerve fiber layer and narrowing of the optic nerve rim as the best available gold standards for diagnosis, and states that combining abnormal structural and functional test results increases diagnostic confidence.
That said, if you had to pick one test that comes closest to a standalone answer, OCT imaging of the optic nerve is what the World Glaucoma Association calls “the best currently available digital imaging instrument for detecting and tracking optic nerve structural damage.” Here’s why, and what the full diagnostic picture looks like.
Why OCT Is the Closest to a Single Best Test
Optical coherence tomography, or OCT, works like an ultrasound but uses light instead of sound. It creates a detailed cross-sectional image of the layers at the back of your eye, measuring the thickness of the nerve fiber layer around your optic nerve down to the micrometer. Because glaucoma destroys these nerve fibers gradually, OCT can detect thinning before you ever notice a change in your vision.
The numbers back this up. In a study comparing OCT devices in early-to-moderate glaucoma, the newer spectral-domain OCT (Cirrus) correctly identified glaucoma in 83% of cases while correctly ruling it out in 88% of healthy eyes when looking at overall nerve fiber thickness. When the criteria were broadened to flag any abnormal section of the nerve layer, sensitivity jumped to 98% with specificity at 80%. At its most sensitive setting, looking at individual clock-hour segments, OCT caught 100% of glaucoma cases, though it also flagged 28% of healthy eyes as potentially abnormal.
This tradeoff between catching every case and avoiding false alarms is exactly why OCT works best as part of a broader workup rather than in isolation. A test that catches everything but over-flags healthy people still needs a second opinion from another type of test.
Visual Field Testing: The Functional Half
While OCT shows what’s happening to the physical structure of your optic nerve, visual field testing (called standard automated perimetry) measures what’s happening to your actual vision. You sit in front of a dome-shaped device, stare at a central point, and click a button whenever you see a small flash of light in your peripheral vision. The machine maps out any blind spots or areas of reduced sensitivity.
Standard automated perimetry is the reference standard for all functional testing in glaucoma. It’s considered mandatory for documenting how much vision loss has occurred and tracking whether it’s getting worse over time. However, it has a significant limitation: structural damage to the nerve fibers usually shows up on OCT before any measurable vision loss appears on a field test. By the time perimetry detects a problem, a meaningful amount of nerve damage has already occurred.
That said, some patients show functional vision changes before structural ones, which is why the World Glaucoma Association recommends measuring both. Relying on only one type of test risks missing the onset of damage in a subset of patients.
Perimetry also becomes less reliable in areas where vision is already significantly damaged. Research shows that in regions where sensitivity has dropped below about 15 decibels, the test results become essentially unreliable, making it harder to track progression in advanced disease.
Eye Pressure: Important but Not Definitive
Many people associate glaucoma with high eye pressure, and measuring intraocular pressure (IOP) with tonometry is a standard part of every glaucoma evaluation. The traditional threshold of 22 mmHg or higher as “abnormal” dates back to a 1958 German population study, where 21 mmHg was simply two standard deviations above the average.
The problem is that up to one-third of glaucoma patients have eye pressure readings within the normal range. This is called normal-tension glaucoma, and it’s common enough that a pressure reading alone would miss a huge portion of cases. On the flip side, many people with elevated pressure never develop glaucoma.
Accuracy of pressure readings also depends on corneal thickness. A cornea that’s 10% thinner or thicker than average can shift the reading by about 3.4 mmHg in either direction. This is especially relevant for people who’ve had laser vision correction, which thins the cornea and can make pressure readings appear falsely low. Some newer devices measure corneal thickness and pressure simultaneously, then automatically adjust the reading.
Gonioscopy: Sorting Out the Type
If other tests suggest glaucoma, gonioscopy helps determine which type you have. Your eye doctor places a special lens directly on your eye (after numbing drops) and uses a slit lamp to view the drainage angle where fluid exits the eye. This is the gold standard for diagnosing angle-closure glaucoma, a type where the drainage channel is physically blocked or narrowed.
No imaging technology has replaced gonioscopy because it provides a full 360-degree view of the drainage angle and can reveal features like abnormal blood vessel growth, scar tissue, or pigment deposits that other scans miss. It’s considered mandatory in the management of all types of glaucoma.
What a Full Glaucoma Workup Includes
A comprehensive glaucoma evaluation typically involves five components: a slit lamp examination of the front of your eye, intraocular pressure measurement, gonioscopy, a dilated examination of the optic disc and nerve fiber layer (often supplemented by OCT imaging), and visual field testing. Your eyes will be dilated, which blurs your near vision for a few hours afterward.
The order matters less than the completeness. Each test answers a different question. Pressure measurement tells you about a key risk factor. Gonioscopy identifies the type. OCT reveals structural nerve damage. Perimetry reveals functional vision loss. The slit lamp exam catches other eye conditions that could mimic or complicate glaucoma. Skipping any one of these leaves a gap in the picture.
Doctors also need to be cautious about over-testing. The World Glaucoma Association specifically warns that using a large number of diagnostic tests increases the chance of false positives and over-diagnosis. The goal is corroboration between structural and functional findings, not simply running every available scan.
AI-Assisted Screening Is Catching Up
Artificial intelligence systems trained on retinal photographs and OCT scans are showing remarkable accuracy. Deep learning algorithms analyzing fundus photographs have achieved sensitivity above 93% and specificity above 95% in clinical validation, routinely matching or exceeding glaucoma subspecialists. In one head-to-head comparison, an AI system detected 94% of glaucoma cases from retinal photos, while three blinded specialists detected only 60% from the same images.
These tools are particularly promising for screening in settings where specialists aren’t available. One system designed for use with a smartphone-based fundus camera achieved 91.4% sensitivity and 94.1% specificity in real-world validation. While AI doesn’t replace the full diagnostic workup, it could dramatically improve early detection, especially in underserved communities.
Who Needs Screening and How Often
The American Academy of Ophthalmology recommends regular eye exams for everyone over 40, with more frequent or earlier exams for those with risk factors. The highest-risk groups include people with a first-degree relative who has glaucoma, those with Type 2 diabetes, African Americans over 50, and Hispanic Americans 65 and older. Medicare covers annual glaucoma screening for these groups.
For African Americans specifically, modeling studies suggest that adding glaucoma screenings at ages 50, 60, and 70 could reduce undiagnosed glaucoma by 33%, visual impairment by nearly 7%, and blindness by about 10%. A universal screening policy for African Americans aged 50 to 59 could cut the lifetime prevalence of undiagnosed glaucoma roughly in half. Because glaucoma causes irreversible damage with no early symptoms, the value of catching it before vision loss begins is enormous.