Glaucoma Tests: What Each Eye Test Checks, and What It Cannot Tell You

Key takeaways

• Glaucoma is a diagnosis of pattern, not of one number. Eye pressure alone neither confirms nor excludes it.
• Most glaucoma is silent early on. Central vision often stays sharp while peripheral vision is quietly lost.
• Normal-tension glaucoma exists. Optic nerve damage can occur with intraocular pressure in the usual range.
• Tests answer different questions. Tonometry measures pressure, OCT measures nerve fibre structure, perimetry measures function, gonioscopy assesses the drainage angle.
• Damage already done cannot be reversed. Treatment aims to slow further loss; results vary depending on individual clinical circumstances.

At a glance

Also known as

• Tonometry: "eye pressure test", "air puff test", IOP check
• Perimetry: visual field test, field charting, "side vision test"
• Ophthalmoscopy / fundus examination: "retina check", optic disc examination
• OCT: optical coherence tomography, "nerve fibre scan", RNFL scan
• Pachymetry: corneal thickness test (CCT)
• Gonioscopy: drainage angle examination

What glaucoma tests are

Glaucoma is a group of optic nerve diseases in which nerve fibres are progressively lost, usually producing a characteristic pattern of visual field loss. Raised intraocular pressure (IOP) is the main modifiable risk factor, but it is a risk factor, not the definition of the disease.

Because of this, glaucoma assessment uses several tests together:

• Tonometry measures intraocular pressure. Goldmann applanation tonometry, performed at the slit lamp after numbing drops, remains the reference method. Non-contact (air puff) tonometry is a convenient screening method but is less precise and readings are more variable.
• Optic disc examination looks directly at the optic nerve head for cupping, rim thinning, disc haemorrhage and asymmetry between the two eyes. Disc photographs help track change over years.
• Perimetry (usually standard automated perimetry) maps functional vision and detects the arcuate and nasal step defects typical of glaucoma.
• Gonioscopy uses a mirrored contact lens to view the drainage angle and classify glaucoma as open-angle or angle-closure. This distinction changes treatment entirely, so gonioscopy is important, not optional.
• OCT measures the retinal nerve fibre layer and macular ganglion cell layer thickness objectively.
• Pachymetry measures central corneal thickness, which influences how tonometry readings should be interpreted and is itself a risk factor.

Why glaucoma tests are done

• To investigate raised eye pressure or a suspicious optic disc found on a routine eye check.
• To establish a baseline against which future change can be judged.
• To distinguish open-angle from angle-closure disease.
• To monitor whether treatment is holding the disease stable.
• To assess eye pain, halos around lights, or unexplained visual field loss.

Who should get tested

• Adults over 40, as part of a periodic comprehensive eye examination.
• Anyone with a first-degree relative with glaucoma: risk is substantially higher, and testing is usually advised earlier and more often.
• People with high myopia or significant hypermetropia (the latter raises angle-closure risk).
• People on long-term steroids: oral, inhaled, nasal, skin creams or steroid eye drops, including over-the-counter drops bought for redness or allergy.
• People with diabetes, previous eye injury, previous eye surgery, or uveitis.
• Anyone with eye pain with halos, rapid vision blurring or a red painful eye: urgent assessment, not routine screening.

What glaucoma tests cannot detect or exclude

• A normal eye pressure does not exclude glaucoma. Normal-tension glaucoma is well recognised, and is relatively common in Asian populations. IOP also fluctuates through the day, so one reading may miss a peak.
• A raised eye pressure does not confirm glaucoma. Many people with ocular hypertension never develop optic nerve damage.
• Perimetry is insensitive early. A substantial proportion of retinal ganglion cells can be lost before a reliable field defect appears. A normal field does not mean the nerve is untouched.
• OCT does not diagnose glaucoma. It reports thickness compared with a normative database. Thinning also occurs in other optic neuropathies, high myopia and retinal disease; thick or oedematous tissue can mask loss.
• Air puff tonometry does not replace applanation. It screens; it does not settle a borderline case.
• No glaucoma test assesses the visual pathway behind the eye. Field loss from stroke, pituitary tumour or optic neuritis needs neurological assessment and imaging.
• Screening cannot recover lost vision or predict with certainty who will progress.
• "Not identified" is not "excluded". If your vision changes after a normal glaucoma assessment, return to your ophthalmologist rather than waiting for the next scheduled review.

Related but different examinations

How to prepare

Preparation is minimal, but instructions differ between eye units and equipment. Follow the instructions your unit gives you, they take precedence over anything written here.

• Bring your current spectacles and prescription, plus previous reports, field printouts and OCT scans. Comparison with old records is often more informative than any single new test.
• List all medicines, including eye drops, inhalers, nasal sprays, skin creams and any AYUSH or over-the-counter preparations. Do not stop or change any prescribed medicine before testing; ask the prescriber.
• Contact lenses usually need to be removed before tonometry, gonioscopy and pachymetry. Some units ask for lenses out for a period beforehand.
• Arrange transport. If drops are used to dilate the pupils, vision blurs and glare increases for several hours. Do not drive or ride a two-wheeler yourself afterwards. Bring sunglasses.
• Plan for time and concentration. Perimetry requires sustained attention; being rested improves reliability.

If you have a history of angle closure

Dilating drops can, rarely, precipitate an acute angle-closure attack in susceptible eyes. Tell the team if you have narrow angles, previous acute glaucoma, or have been advised against dilation. Clinicians weigh this before dilating.

Pregnancy and breastfeeding

Tonometry, OCT, perimetry and pachymetry themselves carry no radiation and are not known to harm pregnancy. Some eye drops are absorbed systemically, so tell the team if you are pregnant or breastfeeding so the choice of drops can be reviewed.

Children

Paediatric glaucoma assessment is a specialist activity. Young children may not cooperate with perimetry or slit-lamp tonometry, and examination under anaesthesia is sometimes needed. Measurements taken under anaesthesia can differ from those taken awake.

Older adults and people with limited mobility or dementia

Perimetry reliability falls with fatigue, tremor, hearing difficulty or cognitive impairment. Structural tests such as OCT and disc examination carry more weight in these situations.

After recent eye surgery or injury

Contact tests such as applanation tonometry, gonioscopy and ultrasound pachymetry may be deferred after recent surgery, corneal ulcer, active infection or an open wound. Tell the team about any recent procedure.

What happens during the tests

• Vision and refraction: letters are read from a chart; a suitable lens correction is set up for perimetry.
• Tonometry: for air puff, you rest your chin and a brief puff of air touches the eye. For applanation, numbing and fluorescein drops are instilled and a small probe gently touches the cornea for a second or two. It is not painful, though it feels odd.
• Slit-lamp and optic disc examination: you sit with chin and forehead supported while a bright light and lens are used. Pupils may be dilated first.
• Gonioscopy: after numbing drops, a small mirrored lens is placed against the eye for a minute or two. Mild pressure sensation is normal.
• Pachymetry: either a brief ultrasound probe touch after numbing drops, or a no-touch optical measurement.
• OCT: you look at a target while the scanner captures images. No contact, no injection.
• Perimetry: one eye is patched; you fix on a central light and press a button whenever you notice a faint spot appear anywhere in your side vision. Each eye commonly takes 5-10 minutes depending on the programme used.

How long the process takes

• Tonometry alone: a couple of minutes.
• Pachymetry or OCT: roughly 5-10 minutes each.
• Gonioscopy: about 5 minutes.
• Perimetry: about 10-20 minutes for both eyes, longer for detailed programmes.
• Full first-visit glaucoma work-up: commonly 1-2 hours including registration, dilation waiting time and consultation.
• Effects of dilating drops: blurred near vision and glare typically for 4-6 hours, occasionally longer in lightly pigmented eyes.

Scheduling and reporting arrangements differ between hospitals and eye clinics.

Understanding your report

No single value on a glaucoma report is a diagnosis. The ophthalmologist who examined you interprets the tests together, in clinical context, and often against your own previous results.

• Intraocular pressure: commonly quoted population reference figures sit in the low-to-high teens, with values above about 21 mmHg conventionally labelled raised. That cut-off is a statistical convention, not a disease boundary. Pressure that is "acceptable" for one person may be damaging for another, which is why clinicians set an individual target pressure.
• Cup-to-disc ratio: a description of how much of the disc is cupped. Large discs naturally have large cups. Asymmetry between the two eyes, thinning of the neuroretinal rim and disc haemorrhages carry more weight than the number alone.
• Perimetry indices: reports show a greyscale map plus summary indices and reliability indices (fixation losses, false positives, false negatives). An unreliable field cannot be interpreted and is often repeated. First fields frequently look worse than reality because of a learning effect.
• OCT printouts: colour-coded green, yellow and red bands compare your measurements with a normative database. A red flag does not mean glaucoma, and green does not mean healthy, these are statistical comparisons with a reference population that may not match your eye.
• Pachymetry: reported in microns. Thinner corneas tend to under-read true pressure on applanation and are themselves associated with higher risk of progression. There is no reliable universal formula to "correct" IOP for corneal thickness.
• Gonioscopy: recorded as open, narrow or closed angle, sometimes with grading and notes on peripheral anterior synechiae or pigmentation.

When results can be misleading

Readings that can be falsely raised

• Thick corneas, corneal scarring or oedema can push applanation readings upward.
• Breath-holding, squeezing the eyelids, a tight collar or pressing on the globe during measurement raises the reading.
• Measurement soon after strenuous exertion, or in certain body positions, can differ from the usual value.
• Air puff tonometry tends to be less reliable at the extremes and can overestimate in some eyes.
• On OCT, epiretinal membrane, disc swelling or vitreous traction can artificially increase measured thickness, masking real loss, or producing confusing "thick" flags.

Readings that can be falsely lowered

• Thin corneas, and eyes after LASIK or other refractive surgery, commonly under-read on applanation, so true pressure may be higher than printed.
• A single morning or evening reading may miss the daily peak; diurnal variation can be substantial.
• Excess fluorescein, an irregular tear film or recent eye rubbing can distort applanation.

Perimetry results that look worse than the true state

• Fatigue, poor concentration, drowsiness or unfamiliarity with the test (learning effect).
• Droopy eyelid, spectacle rim artefact, uncorrected refractive error or an inappropriate trial lens.
• Cataract, dry eye or corneal disease reducing overall sensitivity.
• Small pupils, including after pilocarpine-type drops.

Scans that misclassify

• High myopia and tilted discs frequently produce red "abnormal" flags on OCT in eyes without glaucoma.
• In advanced glaucoma, OCT reaches a measurement floor and can no longer track further deterioration.
• Poor scan quality from dry eye, cataract or eye movement produces unreliable segmentation.

Risks and safety

Non-contact tests (air puff tonometry, optical pachymetry, OCT, fundus photography, perimetry)

These do not touch the eye and involve no radiation or injection. The usual complaints are startle from the air puff, glare from bright flashes, and tiredness or neck ache during perimetry.

Contact tests (applanation tonometry, gonioscopy, ultrasound pachymetry)

These touch the anaesthetised cornea. Uncommon issues include transient stinging, a small corneal abrasion, allergic reaction to drops and, rarely, infection transmission if instruments are not properly disinfected. Tell the team about contact lens wear, corneal disease or recent eye surgery.

Dilating drops

Blurred near vision, glare and light sensitivity for several hours; occasionally a rise in heart rate or flushing, more noticeable in young children and older adults. In eyes with narrow angles there is a small risk of precipitating acute angle closure, which is why gonioscopy usually precedes dilation when angles are suspect.

Results and experiences vary depending on individual clinical circumstances.

Special situations

People with diabetes

Diabetes raises the risk of several eye problems, and neovascular glaucoma can follow advanced retinopathy. A retinopathy screen and a glaucoma assessment are separate examinations; having one done does not cover the other.

People on steroids

Steroid-induced pressure rise is common and often silent. This includes eye drops bought without prescription for redness or allergy, a frequent problem in India. Do not stop a prescribed steroid yourself, tell the prescribing doctor that your eye pressure needs monitoring.

High myopia

Highly myopic eyes carry a higher glaucoma risk, yet are also the hardest to assess: discs are tilted, OCT normative databases fit poorly, and fields can be abnormal for non-glaucomatous reasons. Serial testing matters more than any single scan.

After cataract or refractive surgery

Cataract surgery can alter both pressure readings and field results. After LASIK or similar procedures, applanation readings typically under-read; make sure the clinic knows about any refractive surgery.

Pregnancy

Intraocular pressure often falls slightly during pregnancy. Women with existing glaucoma should discuss drop safety with both the ophthalmologist and the obstetrician rather than adjusting treatment themselves.

Older adults

Coexisting cataract, dry eye and reduced concentration all degrade test quality. Interpretation leans more heavily on the disc examination and on comparison with earlier records.

Glaucoma testing in the Indian context

• Angle-closure disease is proportionally more common in Indian and other Asian populations than in Western populations, which makes gonioscopy an essential part of assessment rather than an add-on.
• Normal-tension glaucoma is well described in Asian eyes, so relying only on an air puff pressure reading at an optical store risks false reassurance.
• Over-the-counter steroid eye drops are widely available and are a recognised cause of preventable pressure rise and optic nerve damage. Steroid-containing drops should be used only under medical supervision with pressure monitoring.
• Late presentation is common. Many people in India are first diagnosed when peripheral vision is already substantially lost, because early glaucoma is painless and central vision stays sharp.
• Terminology you will hear locally: "aankh ka pressure", "kala motia" for glaucoma and "safed motia" for cataract, these are different conditions with different treatments, and the two are often confused.
• Access: tonometry and fundus examination are widely available even in smaller towns; OCT and automated perimetry are concentrated in larger hospitals and dedicated eye institutes, so follow-up is sometimes best planned at one centre for consistency.

Cost and insurance in India

Charges vary widely between cities, between government, trust-run and private facilities, and between a single test and a bundled glaucoma work-up. Ask for an itemised estimate before testing.

Factors that affect what you pay:

• Whether a single test or a full work-up (tonometry, gonioscopy, pachymetry, OCT, perimetry, disc photographs) is done.
• Whether one eye or both eyes are tested.
• Type of facility, charitable eye hospital, government hospital, standalone clinic or corporate hospital.
• City tier and whether consultation fees are billed separately.
• Repeat testing: glaucoma monitoring is lifelong, so the recurring annual cost matters more than the one-off figure.
• Cost of medication and follow-up, which usually exceeds the cost of the tests themselves over time.

Insurance: health insurance in India generally reimburses diagnostic tests when they are part of an admitted or day-care procedure. Purely outpatient diagnostics are often not covered unless you hold an OPD rider or a corporate plan that includes them. Glaucoma surgery and laser procedures are more commonly covered, subject to waiting periods and pre-existing disease clauses. Check with your insurer before the visit.

Myths and facts

Frequently asked questions

Which single test confirms glaucoma?

None does. Glaucoma is diagnosed from a pattern: optic disc appearance, retinal nerve fibre thinning on OCT, matching visual field loss, the drainage angle on gonioscopy, and pressure readings interpreted with corneal thickness. Ophthalmologists often need repeat testing over months before confirming or ruling out the condition.

Is the air puff test enough for screening?

Air puff tonometry only estimates eye pressure, and does so less precisely than applanation. Because glaucoma can occur at normal pressure, an air puff reading alone can falsely reassure. A proper assessment also requires examination of the optic nerve, ideally with dilated pupils.

Do glaucoma tests hurt?

Generally no. The air puff startles briefly. Applanation tonometry, gonioscopy and ultrasound pachymetry are done after numbing drops, which sting for a second and then remove sensation. Mild scratchiness afterwards is common and settles within hours; persistent pain should be reported.

How often should glaucoma testing be repeated?

Frequency depends on risk. People without risk factors are usually advised a comprehensive eye examination periodically from around age 40. Those with a family history, high myopia, steroid use, diabetes or an existing diagnosis need more frequent review, sometimes several times a year. Your ophthalmologist sets the interval.

Why does my visual field report look different each time?

Perimetry is subjective and variable. Fatigue, concentration, pupil size, cataract, spectacle rim position and practice effects all shift results. Because of this, a single abnormal field is rarely acted on; clinicians look for a consistent, repeatable defect matching the optic disc appearance.

Will my eyes be dilated, and can I drive afterwards?

Dilation is often needed for a proper optic disc examination. Drops blur near vision and cause glare for several hours. Do not drive or ride a two-wheeler yourself; arrange transport and bring sunglasses. Tell the team if you have previously had narrow angles or acute glaucoma.

Can glaucoma be prevented by testing?

Testing does not prevent glaucoma from developing. What testing can do is detect optic nerve damage earlier, allowing treatment that lowers pressure and slows further loss. Results vary depending on individual clinical circumstances, and some people progress despite well-controlled pressure.

My OCT report shows red areas. Do I have glaucoma?

Not necessarily. Red on an OCT printout means your measurement falls outside the normative database range. High myopia, tilted discs, small discs and scan artefacts all produce red flags without glaucoma. The report must be read alongside the clinical examination and visual fields by your ophthalmologist.

My tests were normal but my vision still feels off. What now?

A normal glaucoma work-up describes what was found on that day; it does not exclude every cause of visual symptoms. Dry eye, cataract, retinal disease and neurological causes can all affect vision. Return to your ophthalmologist and describe the symptom clearly rather than waiting for the next scheduled review.

Sources

• World Health Organization, World Report on Vision and global eye health data on glaucoma as a cause of irreversible blindness.
• American Academy of Ophthalmology, Preferred Practice Pattern guidelines on Primary Open-Angle Glaucoma and Primary Angle Closure Disease.
• European Glaucoma Society, Terminology and Guidelines for Glaucoma.
• National Institute for Health and Care Excellence (NICE), guideline on glaucoma diagnosis and management.
• All India Ophthalmological Society and Glaucoma Society of India, Indian practice guidance on glaucoma evaluation.
• National Programme for Control of Blindness and Visual Impairment, Ministry of Health and Family Welfare, Government of India.