Glaucoma Testing in San Francisco: What the Tests Check and Why Pressure Alone Is Not Enough

Most people picture one thing when they hear glaucoma test: the puff of air. You rest your chin on the machine, it startles you, someone says your pressure looks fine, and you move on with your day.

Here's the part that rarely gets explained. Your eye pressure can read perfectly normal and you can still have glaucoma. Population studies have found that about half of people with open angle glaucoma have pressure below the number usually used as the screening cutoff. A pressure check is one piece of information. On its own it can't tell you whether you have glaucoma.

The short answer: Glaucoma testing is not a single test. It is a group of them. Eye pressure measurement, a close look at the optic nerve through a dilated pupil, imaging that measures the thickness of the nerve fiber layer, and a visual field test that maps your side vision. The American Academy of Ophthalmology states plainly that a glaucoma screening which only checks eye pressure is not enough to find glaucoma. Damage from glaucoma cannot be reversed once it happens, which is why the testing is built around catching it before you can feel anything.

This guide comes from Dr. Michelle Blas, OD at Eyes in Disguise Optometry on Union Street in Cow Hollow. Below is what each test actually does, how often adults need to be checked, who carries higher risk, and the one version of glaucoma that is a same day emergency.

Why the Pressure Reading Alone Cannot Rule Out Glaucoma

Glaucoma is damage to the optic nerve, the cable that carries what you see from your eye to your brain. High pressure inside the eye is the most treatable cause of that damage, which is why pressure gets measured at every visit. But pressure is the risk factor, not the disease.

The Baltimore Eye Survey found that more than half of glaucomatous eyes had a screening pressure below 21 mmHg, the number commonly treated as the dividing line. The American Academy of Ophthalmology puts the practical consequence in the 2020 edition of its Primary Open-Angle Glaucoma Preferred Practice Pattern: using a pressure above 21 mmHg as the test, tonometry identified only 47.1 percent of open angle glaucoma cases. That edition describes the 21 mmHg line as arbitrary and notes the poor predictive value of any single pressure cutoff for screening.

There's a name for glaucoma that develops at normal pressure. It's called normal tension glaucoma, and the Academy's current Preferred Practice Pattern, the 2025 edition, confirms that many people with open angle glaucoma have untreated pressure consistently within the normal range. The National Eye Institute makes the same point from the other direction: some people with high eye pressure never develop glaucoma at all.

So the pressure number matters, and lowering it helps even in normal tension cases. It simply can't do the job by itself.

What a Real Glaucoma Evaluation Includes

The Academy's Preferred Practice Pattern for glaucoma suspects, 2025 edition, lists the components of a workup as gonioscopy, pachymetry, tonometry, perimetry, clinical examination of the optic nerve, and ocular imaging. In plain language, here is what each of those is and why it exists.

Tonometry, the pressure measurement

This measures the pressure inside your eye. It is done either with the air puff you already know about, or with applanation, where numbing drops go in first and a small probe gently touches the surface of the eye. Applanation is generally the more precise of the two. Because readings vary between instruments and across the day, the American Optometric Association's clinical guidance on comprehensive adult eye examination advises considering more than one reading with the same instrument to reduce measurement error. One reading on one morning doesn't settle the question.

Optic nerve examination, usually through a dilated pupil

Your optometrist looks directly at the head of the optic nerve for the specific pattern glaucoma leaves behind: the central cup enlarging, the rim of nerve tissue around it thinning, and sometimes small hemorrhages at the disc margin. A cup to disc ratio above 0.5 is frequently associated with glaucoma, though it has to be read against the size of your particular nerve.

This is where dilation earns its place. The American Optometric Association's clinical guidance on comprehensive adult eye examination states that pharmacologic dilation of the pupil is generally required for a thorough stereoscopic evaluation of the optic nerve and peripheral retina. The National Eye Institute is blunter: a comprehensive dilated eye exam is the only way to catch glaucoma early. If you have risk factors, expect drops and plan your day accordingly.

OCT, imaging the nerve fiber layer

Optical coherence tomography uses light waves to take cross sectional images of the retina and optic nerve. It lets your doctor see the individual layers, measure the thickness of the nerve fiber layer, and compare that measurement against your own scans from previous years.

The value here is timing. The 2020 edition of the Academy's glaucoma Preferred Practice Pattern notes that some patients show structural changes in the optic nerve head and nerve fiber layer before any functional change shows up. Published research found that at least 25 to 35 percent of retinal ganglion cells are already lost by the time an automated visual field test reads as abnormal. Structure changes first. Function follows. Imaging is how the first part gets caught.

Visual field testing, mapping your side vision

You look straight ahead at a fixed point while small lights appear at the edges of your vision, and you signal each time you see one. It maps where your peripheral vision is intact and where it is not.

This matters because open angle glaucoma takes peripheral vision first, and your brain fills in the gaps so convincingly that people routinely lose a significant amount before noticing anything. The Academy describes visual fields and OCT as complementary, each carrying information the other can't supply. One measures what the nerve looks like, the other measures what it can still do, and both are needed to track whether the disease is moving.

Pachymetry, measuring corneal thickness

A quick measurement of how thick the clear front surface of your eye is. It does two jobs.

First, it corrects the pressure reading. As the Academy's 2020 glaucoma Preferred Practice Pattern explains, applanation tonometry works by measuring resistance to indentation, so corneal thickness introduces error. A thicker than average cornea tends to make pressure read higher than it truly is, and a thinner than average cornea tends to make it read lower. Without knowing your corneal thickness, a reassuring number can be false reassurance.

Second, thin corneas are an independent risk factor in their own right. The Ocular Hypertension Treatment Study found central corneal thickness to be a powerful predictor of developing open angle glaucoma, with greater risk in eyes measuring under 555 micrometers than in eyes at 588 or above.

Gonioscopy, checking the drainage angle

A special mirrored contact lens is rested against the eye so the doctor can see the angle where fluid drains out. This determines whether your drainage angle is open or narrow, which is what separates the slow, silent form of glaucoma from the sudden, painful one. The Academy's 2020 glaucoma Preferred Practice Pattern states that diagnosing open angle glaucoma requires careful evaluation of this angle in order to rule out angle closure and other causes of raised pressure.

How Often Should Adults Be Checked?

Two national bodies publish adult exam schedules, and they do not fully agree. Both are current, so here they are side by side.

The American Academy of Ophthalmology, in its Comprehensive Adult Medical Eye Evaluation Preferred Practice Pattern, 2025 edition, recommends these intervals for adults with no symptoms and no risk factors:

• Under 40: every 5 to 10 years
• 40 to 54: every 2 to 4 years
• 55 to 64: every 1 to 3 years
• 65 and older: every 1 to 2 years

The American Optometric Association, in the second edition of its Comprehensive Adult Eye and Vision Examination guideline released in 2023, recommends an annual comprehensive eye and vision examination for every adult age band, with more frequent evaluation when risk factors are present. Note that this replaced the older every two years guidance from the 2015 first edition, which still circulates widely online.

Where both agree completely is on risk. The Academy's own guidance calls for increased frequency of comprehensive eye evaluations in adults who carry risk factors for open angle glaucoma. If anything in the next section describes you, treat the age based intervals as a starting point, and annual is the sensible answer.

Who Carries Higher Risk

The Academy's current Preferred Practice Pattern, the 2025 edition, lists the established risk factors for developing open angle glaucoma. The family history and age points below, and the Academy's own figure on blindness prevalence, are set out in detail in the 2020 edition of the same document. The remaining figures are attributed to their own sources in each bullet. The ones that matter most for deciding when to get checked:

• Family history. The strongest modifier on the list. The Rotterdam study found the odds of having open angle glaucoma were 9.2 times higher for people with a confirmed first degree relative, meaning a parent or sibling. US studies put the sibling figure closer to three and a half times. If a parent or sibling has glaucoma, tell your optometrist before the exam starts.
• Age. Older age raises both the likelihood of having glaucoma and the likelihood of it progressing.
• Race and ethnicity. Prevalence is substantially higher among Black and Hispanic or Latino adults. A 2024 analysis in JAMA Ophthalmology estimated a prevalence of 3.15 percent among Black adults compared with 1.42 percent among White adults. The Academy notes that blindness from glaucoma is at least six times more prevalent in African Americans.
• Diabetes. A 2014 meta analysis of cohort studies published in PLoS One found a 40 percent higher relative risk of open angle glaucoma with diabetes. If you have diabetes, your annual diabetic eye exam is already the right vehicle for this.
• Nearsightedness. Myopia is an established risk factor. An analysis of California Medicare data published in 2023 found roughly 2.4 times the odds of open angle glaucoma in beneficiaries with myopia.
• Steroid use. Steroid eye drops in particular can raise eye pressure. If you are on a steroid for any condition, your eye pressure should be monitored while you take it.
• Thin corneas. See pachymetry above. This is one you can't know without being measured.
• A past eye injury. Worth knowing that this one has a long tail. Glaucoma after trauma may appear immediately or develop months to decades later, often long after the injury itself has been forgotten. If you were ever hit hard in the eye, it belongs in your history no matter how long ago it was.

Can an Optometrist Diagnose and Treat Glaucoma in California?

Yes, within a defined scope. California Business and Professions Code section 3041 allows an optometrist holding the state's glaucoma certification to prescribe topical and oral antiglaucoma agents for the medical treatment of primary open angle, exfoliation, pigmentary and steroid induced glaucomas in patients 18 years of age and over. The California State Board of Optometry describes the same certification in plainer terms: the optometrist is certified to diagnose and treat primary open angle glaucoma in patients over the age of 18.

Earning that certification requires an active license in good standing, prior therapeutic certification, and a didactic course of no less than 24 hours in the diagnosis, treatment and management of glaucoma, along with supervised case management. The Board also expects timely referral to an ophthalmologist when that is the right call.

Two honest caveats. Angle closure glaucoma is not within that medical treatment scope and is handled as an emergency, described next. And glaucoma that is advancing despite treatment, or that needs laser or surgery, belongs with a glaucoma specialist. Detection, monitoring and medical management of open angle glaucoma in adults can happen at your optometrist's office.

The Version That Is a Same Day Emergency

Everything above describes open angle glaucoma, which is slow and painless. Angle closure glaucoma is the opposite. Drainage is blocked suddenly, pressure climbs fast, and it announces itself.

The American Academy of Ophthalmology lists the warning signs as:

• Vision that goes suddenly blurry
• Severe pain in the eye or forehead
• Headache
• Nausea or vomiting
• Rainbow colored rings or halos around lights
• A red eye

The Academy calls this a true eye emergency. The National Eye Institute instructs anyone with these symptoms to go to a doctor or an emergency room immediately. Don't wait for a scheduled appointment, and don't wait to see whether it passes overnight. Go to the nearest emergency room. If we are open, call us at (415) 474-5321 on your way. You can read more about urgent eye care and what counts as an emergency.

What Glaucoma Testing Looks Like at Our Practice

We perform glaucoma screening for every patient using tonometry to measure eye pressure and examination of the optic nerve. If the results warrant it, we recommend further testing or refer to a glaucoma specialist.

Beyond the baseline screening, our Cow Hollow office uses OCT imaging for cross sectional analysis of the retina and optic nerve, and widefield retinal imaging to document the back of the eye so this year's scan can be compared against last year's. Comparison over time is the entire point. A single set of images tells your doctor what your nerve looks like today. A series tells them whether it is changing.

Both of our optometrists hold California's glaucoma certification, which is the credential the state requires in order to diagnose and medically manage open angle glaucoma. Not every optometrist carries it, and it is worth asking about wherever you go. Glaucoma management is one of Dr. Blas's named specialties, alongside primary and emergency eye care, diabetic eye disease and specialty contact lens fitting. Dr. Nina Stroud completed a residency in ocular disease at the VA Greater Los Angeles Healthcare System and has advanced training in glaucoma.

Glaucoma evaluation happens inside a comprehensive eye exam, so you don't need to book anything separate. If you have a family history of glaucoma or any of the other risk factors above, say so when you book, so the right amount of time is set aside.

The Part Worth Acting On

Two facts, taken together, make the case better than any argument could.

The National Eye Institute states that glaucoma usually has no symptoms at first, and that half of people who have it do not know they have it. It also states that treatment will not undo damage to your vision, but it can stop it from getting worse.

The damage is permanent and the warning is silent, so the only variable you control is how early it gets found. Testing is the only thing that moves it.

Frequently Asked Questions

What is the test for glaucoma?

There is no single glaucoma test. A proper evaluation combines eye pressure measurement (tonometry), examination of the optic nerve through a dilated pupil, OCT imaging that measures the thickness of the nerve fiber layer, and visual field testing that maps peripheral vision. Corneal thickness measurement and examination of the drainage angle are added when they are needed. The American Academy of Ophthalmology states that a screening which only checks eye pressure is not enough to find glaucoma.

Can you have glaucoma with normal eye pressure?

Yes. It is called normal tension glaucoma and it is common. The Baltimore Eye Survey found more than half of glaucomatous eyes had a screening pressure below 21 mmHg, and the American Academy of Ophthalmology confirms that many people with open angle glaucoma have untreated pressure consistently in the normal range. Lowering pressure still helps these patients. This is the single best reason not to treat a normal pressure reading as an all clear.

Does the glaucoma test hurt?

No. The air puff version of the pressure test startles people more than it hurts. The applanation version uses numbing drops first, so you feel pressure but not discomfort. Dilating drops sting for a few seconds and then blur your near vision and increase light sensitivity for several hours. Visual field testing and OCT imaging involve no contact with the eye at all.

How often should adults be tested for glaucoma?

Glaucoma evaluation is part of a comprehensive eye exam, so the question is really how often you need that exam. The American Academy of Ophthalmology recommends every 5 to 10 years under 40, every 2 to 4 years from 40 to 54, every 1 to 3 years from 55 to 64, and every 1 to 2 years at 65 and older for adults with no risk factors. The American Optometric Association recommends annually for all adults. Both bodies agree that risk factors mean more frequent exams.

Do I need to be dilated for a glaucoma exam?

For a thorough look at the optic nerve, usually yes. The American Optometric Association's clinical guidance states that dilation is generally required for a proper stereoscopic evaluation of the optic nerve and peripheral retina, and the National Eye Institute describes the comprehensive dilated eye exam as the way glaucoma gets caught early. Plan on blurred near vision and light sensitivity for a few hours afterward, and bring sunglasses or arrange a ride if that will affect your day.

Can an optometrist diagnose and treat glaucoma in California?

Yes, with the state's glaucoma certification. California Business and Professions Code section 3041 permits a certified optometrist to prescribe topical and oral antiglaucoma medication for primary open angle, exfoliation, pigmentary and steroid induced glaucoma in patients 18 and over. Not every optometrist holds that certification, so it is a fair question to ask before you book. Both optometrists at Eyes in Disguise hold it. Angle closure glaucoma is handled as an emergency, and cases needing laser or surgery are referred to a glaucoma specialist.

Why do I sometimes read that glaucoma screening is not recommended?

This comes from the US Preventive Services Task Force, and it is widely misquoted. Its 2022 statement concludes that the current evidence is insufficient to assess the balance of benefits and harms of screening for open angle glaucoma in asymptomatic adults 40 or older. That is a Grade I, which means the evidence is lacking or conflicting, not that screening was found to be unhelpful. It is not a recommendation against testing. Its subject is population screening of asymptomatic adults in primary care, where the Task Force notes these tests are not commonly available, rather than the comprehensive eye exams that eye doctors perform. The American Academy of Ophthalmology and the American Optometric Association both recommend regular comprehensive eye exams that include glaucoma evaluation.

Can vision lost to glaucoma be restored?

No. The National Eye Institute states that treatment will not undo damage to your vision, but it can stop it from getting worse. Once the optic nerve is damaged, that vision does not come back. Treatment, usually pressure lowering drops, is aimed entirely at protecting the sight you still have. This is why finding it before symptoms appear matters so much.

Where can I get glaucoma testing in San Francisco?

Eyes in Disguise Optometry provides glaucoma screening with eye pressure measurement and optic nerve examination for every patient, supported by OCT and widefield retinal imaging, at 2133 Union Street in Cow Hollow. We serve patients from the Marina, Pacific Heights, Russian Hill and throughout San Francisco. Call (415) 474-5321 or book an appointment online. If results warrant it, we will recommend further testing or refer you to a glaucoma specialist.

Book Your Eye Exam

If a parent or sibling has glaucoma, if you have diabetes, if you are over 40, or if you simply cannot remember the last time anyone looked at your optic nerve, that's your answer about whether you're due.

Our comprehensive eye care team handles glaucoma evaluation as part of routine care. If you want the longer view on why exams matter beyond glaucoma, read why regular eye exams matter more than you think.

Call (415) 474-5321 or book online. Mention your family history when you book.

---

This article is for general education and is not a substitute for an examination or medical advice. If you have glaucoma or have been told you are a glaucoma suspect, follow the monitoring schedule your own doctors set for you.

Sources: American Academy of Ophthalmology, Primary Open-Angle Glaucoma Preferred Practice Pattern, 2020 and 2025 editions, and Primary Open-Angle Glaucoma Suspect Preferred Practice Pattern, 2025 edition. American Academy of Ophthalmology, Comprehensive Adult Medical Eye Evaluation Preferred Practice Pattern, 2025 edition. American Optometric Association, Evidence-Based Clinical Practice Guideline for the Comprehensive Adult Eye and Vision Examination, second edition, 2023. National Eye Institute, "Glaucoma," nei.nih.gov. US Preventive Services Task Force, "Primary Open-Angle Glaucoma: Screening," 2022. Sommer A, Tielsch JM, Katz J, Quigley HA, et al., Baltimore Eye Survey, Archives of Ophthalmology, 1991. Gordon MO et al., Ocular Hypertension Treatment Study, Archives of Ophthalmology, 2002. Ehrlich JR et al., "Prevalence of Glaucoma Among US Adults in 2022," JAMA Ophthalmology, 2024. Kerrigan-Baumrind LA, Quigley HA, et al., Investigative Ophthalmology and Visual Science, 2000. Zhou M, Wang W, Huang W, Zhang X, PLoS One, 2014. Yao M, Kitayama K, Yu F, Tseng VL, Coleman AL, JAMA Ophthalmology, 2023. Ng JK, Kaur K, "Traumatic Glaucoma," and "Glaucoma," StatPearls, National Center for Biotechnology Information. California Business and Professions Code section 3041; California State Board of Optometry.

---

About the author: Dr. Michelle Blas, OD, is an optometrist at Eyes in Disguise Optometry in San Francisco's Cow Hollow neighborhood. A graduate of the SUNY College of Optometry and a member of the American Optometric Association, she specializes in primary and emergency eye care, glaucoma and diabetic eye disease management, and advanced contact lens fittings.

Eyes in Disguise Optometry, 2133 Union Street, San Francisco, CA 94123. (415) 474-5321.