OCT Eye Scan: What It Detects and Why Surgeons Rely on It
An OCT eye scan (optical coherence tomography) is a quick, painless, no-touch scan that uses light to take cross-section pictures of the back of your eye: the retina, the macula and the optic nerve. It can show swelling, fluid, thinning and scarring that a normal eye examination cannot see. At Laser Istanbul Clinic it is part of the pre-operative examination for lens, cataract, retina and glaucoma patients, and your surgeon reads the scan with you.
An OCT eye scan uses light waves to produce detailed cross-section images of your retina and optic nerve. It detects macular degeneration, diabetic macular oedema, macular holes, epiretinal membranes and glaucoma damage, often before you notice a change in your vision. It touches nothing, uses no radiation and takes a few minutes. Doç. Dr. Muhammet Derda Özer, Prof. Dr. Ahmet Satıcı and Op. Dr. Aysan Bayram use it to diagnose, plan surgery and monitor treatment.
What is an OCT eye scan and what does it measure?
An OCT eye scan is an imaging test that uses light waves to take cross-section pictures of your retina. The American Academy of Ophthalmology (AAO) describes OCT as a non-invasive test that lets the doctor see each of the retina's distinct layers and the nerve fibre layer, and measure how thick they are. Think of it as a microscopic slice through the back of the eye, rather than a flat photograph of its surface.
EyeWiki (AAO) explains that OCT uses low-coherence light and a technique called interferometry to build a cross-sectional map of the retina, from the vitreous gel down to the choroid. The original technology was accurate to within about 10–15 microns. Modern spectral-domain devices capture roughly 20,000–40,000 depth scans a second with a resolution of about 3–5 microns, and swept-source devices are faster still. That is why a single capture takes seconds and shows detail that is invisible through the pupil.
What an OCT eye scan measures:
- Retinal thickness, especially at the macula, the small central area that gives you sharp reading and driving vision
- The layers of the retina, including whether fluid, scarring or deposits have disturbed them
- The retinal nerve fibre layer and optic nerve head, the thickness and shape of which change in glaucoma
- The gel–retina interface, where a membrane or traction can pull on the macula
- Blood flow, with OCT angiography (OCTA). The AAO notes that OCTA shows the blood vessels in and under the retina much as fluorescein angiography does, but without a dye injection
What can an OCT scan detect?
An OCT scan can detect conditions of the macula, retina and optic nerve, often before you notice any change in your vision. The AAO lists macular holes, macular pucker, macular oedema, age-related macular degeneration, glaucoma, diabetic retinopathy, central serous retinopathy, geographic atrophy, vitreous traction, abnormal blood vessels, blood-vessel blockage and drusen. This is what the main findings look like.
| Condition | What the OCT scan shows | Where it leads |
|---|---|---|
| Age-related macular degeneration (AMD) | Drusen, fluid or abnormal vessels under the retina, thinning of the macula | Monitoring, or treatment chosen by the retina surgeon |
| Diabetic macular oedema | Swelling of the macula, measured in microns | Treatment chosen by the retina surgeon |
| Macular hole | A gap in the centre of the macula | Retina surgery (vitrectomy) is considered |
| Epiretinal membrane (macular pucker) | A thin scar film wrinkling the macula | Monitoring, or surgery if vision is affected |
| Vitreomacular traction | Gel pulling on the macula | Monitoring or surgery |
| Glaucoma | Thinning of the nerve fibre layer and changes at the optic nerve head | Pressure-lowering treatment, laser or surgery |
| Central serous retinopathy | Fluid under the macula | Your surgeon advises on observation or treatment |
The NHS has recognised this value. A 2024 NHS screening blog reports that OCT was being phased into the diabetic eye screening follow-up pathway in England, because an OCT scan gives a detailed cross-section through the retina and can identify some features of treatable diabetic eye disease more accurately than a standard retinal photograph.
Can an OCT eye scan detect a tumour?
OCT can show the effects of an eye tumour, but it is not the test that diagnoses one. A 2015 review of enhanced-depth OCT in the Indian Journal of Ophthalmology found that it can show features that help separate a harmless choroidal naevus (freckle) from a small melanoma. "Shaggy" photoreceptors over the lesion were seen in 49% of melanomas and in none of the naevi studied. The same review notes that OCT cannot adequately image peripheral tumours or lesions larger than 5 mm across. A suspicious lesion therefore needs a full examination and other imaging, such as eye ultrasound. Doç. Dr. Özer, who treats ocular tumours, reviews these cases.
Can an OCT scan detect MS?
OCT cannot diagnose multiple sclerosis (MS), but it can show damage that MS leaves in the optic nerve. A 2014 review in Degenerative Neurological and Neuromuscular Disease reports that after an attack of optic neuritis the retinal nerve fibre layer typically thins by about 20 microns, and that eyes of people with MS but no previous optic neuritis are about 10 microns thinner than healthy controls. The authors stress that OCT is a complement to MRI, not a replacement, and that it is not yet a standard way of diagnosing MS. A neurologist makes that diagnosis. We can scan the optic nerve and send the report to your doctor.
Do I need an OCT scan? Why it matters before surgery
You need an OCT scan if you have a retinal or macular condition, glaucoma or suspected glaucoma, diabetes affecting the eyes, or blurred or distorted central vision with no clear cause. You also benefit from one before cataract or lens surgery, because a scan can reveal macular problems that would otherwise be missed.
1. It checks the macula before cataract and lens surgery. A cloudy lens can make the retina hard to examine. An AAO EyeNet report (2020) states that OCT detects subtle macular changes in 9–30% of retinas that look normal on examination. A cost-effectiveness study by Leung, Gibbons and Koch (Ophthalmology, 2020) estimated that about 20.5% of cataract patients have macular disease, and that 11% of these may be missed on the initial clinical examination. The AAO article quotes a surgeon: "a premium IOL requires a premium macula". If a scan shows macular disease, your surgeon can advise against a multifocal lens, or set realistic expectations before you choose one.
2. It is central to glaucoma care. The NICE glaucoma guideline (NG81) recommends optic nerve assessment with slit-lamp examination plus OCT or an optic nerve head image where available, and a baseline image of the optic nerve at diagnosis, such as a stereoscopic photograph or an OCT scan. NICE also notes that OCT alone is not enough to screen for or confirm glaucoma. It works as part of a series of tests that includes eye pressure and visual field tests.
3. It guides retina surgery. A scan shows whether a macular hole is present, whether a membrane is pulling on the macula and how much fluid there is, which helps the surgeon plan a vitrectomy.
4. It tracks treatment. Repeated scans show whether swelling is shrinking or whether glaucoma damage is progressing, so treatment can be adjusted.
What to expect: the OCT eye scan procedure
Before the scan
- Bring your glasses, your current prescription and any previous OCT scans or retina reports. Earlier scans are valuable, because comparing them shows change over time.
- Dilating drops are sometimes used. The AAO says drops may be used to widen the pupil, but dilation is not always necessary. If you are dilated, your eyes may stay sensitive to light for several hours, so bring sunglasses and avoid driving until your vision is comfortable again.
During the scan
- You sit at the machine with your chin on a rest and your forehead against a support.
- You look at a fixed light or target inside the device while the scanner sweeps a beam of light across your eye.
- You keep your eye open and try not to blink during each capture, which lasts a few seconds.
- The scan is repeated for the other eye, and sometimes again if you blinked or moved.
How long does an OCT scan take? The AAO gives about 5–10 minutes for the scanning visit, and each individual capture takes seconds. The scan does not touch your eye, and there is nothing to feel. It is one short part of a full examination.
After the scan: there are no after-effects from the scan itself. If you had dilating drops, your vision will be blurred and light-sensitive for some hours. Your surgeon goes through the images with you at the same appointment.
Normal and abnormal OCT scan results: how they are read
OCT scan interpretation is a specialist skill. Your surgeon looks at the cross-section images, the thickness numbers, the colour maps and the two eyes side by side, together with your symptoms and examination. A scan is a piece of evidence, not a verdict.
A normal OCT scan shows smooth, well-defined retinal layers, a normal dip (the fovea) at the centre of the macula and no fluid, holes or membranes. Thickness numbers depend on the machine. A study comparing devices (Gella and colleagues, Saudi Journal of Ophthalmology, 2014) lists average central macular thickness values from about 229 to 260 microns depending on the device, and concludes that different OCT instruments cannot be used interchangeably. This is why follow-up scans are best done on the same machine.
An abnormal OCT scan may show:
- Thickening and cysts in the macula, which means fluid or swelling
- Thinning, which may mean loss of retinal cells
- Fluid under the retina, or a raised area of the layers
- A hole, membrane or traction at the macula
- A thin nerve fibre layer, which can point to glaucoma or other optic nerve disease
Colour codes in nerve fibre reports. Reports often colour the measurements against a reference database. In one widely used scheme, green means within the normal range (between the 5th and 95th percentiles), yellow means within the thinnest 5% and red within the thinnest 1% of the reference group. A study in Ophthalmology Glaucoma (2021) treats yellow and red as abnormal, and also showed that healthy eyes can get unexpected colour codes. A red or yellow result is therefore a reason for a closer look, never a diagnosis on its own.
Is an OCT scan safe? Risks and limits
An OCT scan is very safe. It uses light, not X-rays, and nothing touches the eye, so there is no radiation. The small things to know are these.
- Dilating drops blur near vision and make you light-sensitive for some hours, as described above.
- Poor image quality. The AAO notes that OCT cannot work properly when light cannot get through, for example with corneal scarring, a dense cataract or bleeding in the vitreous gel. EyeWiki adds that patient movement degrades the image. In these cases your surgeon uses another test, such as eye ultrasound.
- Artefacts and false alarms. A 2023 study in the Journal of Glaucoma found OCT measurement artefacts in 51.9% of highly short-sighted eyes, against 18.6% of other eyes, which can make a glaucoma assessment misleading. In highly myopic eyes the nerve fibre numbers must be read with caution.
- Not a diagnosis by itself. OCT shows structure, not how well the eye functions. It is combined with your vision, eye pressure, visual field and, when needed, angiography.
Is an OCT scan worth it? Price and NHS access
An OCT eye scan is worth it if you have, or may have, a retinal, macular or optic nerve problem, or if you are planning cataract, lens or retina surgery. It is quick, painless and finds changes that other tests can miss, and the evidence above shows it can change decisions before premium lens surgery. If you have no symptoms, no family history of glaucoma and no diabetes, it is a reasonable extra rather than an urgent need. Ask your eye professional why they recommend it.
OCT eye scan cost at Laser Istanbul Clinic. OCT is included in every treatment price as part of your pre-operative examination and scans with your surgeon. If you only want an OCT, for example a retina or glaucoma check or a second opinion, a standalone examination with OCT costs {{£ price for a standalone OCT scan}}. Hotel and airport transfers are not included and are a separate charge if you would like us to arrange them.
Is an OCT scan free on the NHS? Hospital eye clinics use OCT as a routine part of NHS care when your condition needs it. At a high-street optician, however, it may not be covered. A 2025 report from Optometry Scotland states that OCT is not currently funded through General Ophthalmic Services, the system that pays for NHS-funded sight tests, and that cost is the main barrier to access. Practices therefore often charge a separate fee, and availability varies by area. Ask your optician what the fee is before the test.
You can also send scans from your own optician or eye clinic for a free initial review before you travel.
Who reads your scans
Your OCT images are read by the surgeon who treats you, not just by a machine. Doç. Dr. Muhammet Derda Özer (Doç. means Associate Professor) is our retina, uveitis and ocular oncology specialist and reviews macular, retinal and tumour-related scans. Prof. Dr. Ahmet Satıcı uses OCT in glaucoma, cataract and premium-lens planning. Op. Dr. Aysan Bayram, our female surgeon, uses it in cataract and retinal disease. Our examination uses {{OCT device models used by your surgeons}}. If your scan shows anything unexpected, your surgeon explains what it means and what the options are, through your personal interpreter if you wish.
Treatments an OCT scan supports
| Treatment | How OCT is used |
|---|---|
| Retina surgery (vitrectomy, retinal detachment, macular hole, membrane) | Diagnoses macular holes and membranes, plans surgery and checks healing |
| Glaucoma treatment and surgery | Measures optic nerve damage, sets a baseline and tracks progression |
| Cataract and lens replacement surgery | Rules out macular disease before a premium lens is chosen |
| Injections and laser for macular disease | Shows fluid and swelling before treatment and how well it responds |
Other tests in your eye examination
An OCT scan is usually done alongside other tests, each answering a different question:
- Corneal topography: maps the shape of the cornea (/en/eye-tests-diagnostics/corneal-topography/)
- FFA (fundus fluorescein angiography): dye imaging of the blood vessels at the back of the eye (/en/eye-tests-diagnostics/fluorescein-angiography/)
- Eye ultrasound (B-scan): imaging when the inside of the eye cannot be seen directly (/en/eye-tests-diagnostics/eye-ultrasound/)
- Visual field test (perimetry): checks your side vision (/en/eye-tests-diagnostics/visual-field-test/)
- Tonometry (eye pressure measurement): essential for glaucoma checks (/en/eye-tests-diagnostics/tonometry/)
- VEP & ERG electrophysiology tests: measure the electrical responses of the retina and visual pathway (/en/eye-tests-diagnostics/vep-erg-tests/)
OCT eye scan: frequently asked questions
An OCT eye scan is a painless, no-touch imaging test that uses light waves to take cross-section pictures of the retina and optic nerve. It lets your surgeon see the retinal layers and measure their thickness. It is used to diagnose and monitor macular and retinal disease and glaucoma.
It can detect age-related macular degeneration, diabetic macular oedema, macular holes, epiretinal membranes, central serous retinopathy, vitreous traction and glaucoma damage to the optic nerve. It can show these changes before they affect your vision noticeably. It does not replace a full eye examination.
You need one if you have a macular, retinal or optic nerve condition, glaucoma or raised eye pressure, diabetes affecting the eyes, or unexplained blurred or distorted central vision. It is also advisable before cataract or premium lens surgery, because it can find macular disease that changes your lens choice.
Yes, when there is a clinical reason for it. Studies show that OCT finds macular changes in 9–30% of retinas that appear normal on examination, and it can change the plan before lens surgery. With no symptoms and no risk factors it is a useful extra, not a necessity.
A normal scan shows smooth, regular retinal layers, a normal central dip and no fluid, holes or membranes. An abnormal scan may show swelling, thinning, fluid, a membrane or a thin nerve fibre layer. Thickness numbers differ between machines, so your surgeon reads them together with your examination, not on their own.
It is very safe. It uses light, not radiation, and nothing touches the eye. The only usual side effect comes from dilating drops, which blur your vision and cause light sensitivity for some hours. The scan can be unclear if you have a dense cataract or corneal scarring, and in very short-sighted eyes it can produce measurement artefacts.
It does not hurt. You rest your chin on a support and look at a light. Each capture takes seconds, and the AAO gives about 5–10 minutes for the scanning visit.
Not always. The AAO says dilation may be used but is not always necessary. If you are dilated, your eyes stay light-sensitive for several hours, so avoid driving until your vision feels normal and bring sunglasses.
In an NHS hospital eye clinic, OCT is part of your care when your condition needs it. At a high-street optician it is not covered by the NHS sight test funding, according to a 2025 Optometry Scotland report, so a fee is often charged. Ask what it costs before you book.
OCT is included in every treatment price at Laser Istanbul Clinic as part of your pre-operative examination. A standalone OCT examination costs {{£ price for a standalone OCT scan}}. Hotel and transfers are extra.
Only in part. OCT can show features that help tell a choroidal melanoma from a harmless naevus, such as "shaggy" photoreceptors over the lesion, but it cannot image large or peripheral tumours well. Diagnosis needs a full examination and other imaging such as eye ultrasound.
Not by itself. OCT can show thinning of the optic nerve fibre layer after optic neuritis, which is common in MS, but MS is diagnosed by a neurologist, usually with MRI. OCT is a complementary test.
Sources for the facts on this page
- American Academy of Ophthalmology, What Is Optical Coherence Tomography?: non-invasive, light waves, retinal layers and nerve fibre layer; conditions detected; OCTA without dye; about 5–10 minutes; dilation not always necessary; limits with corneal scarring, dense cataract, vitreous bleeding. https://www.aao.org/eye-health/treatments/what-is-optical-coherence-tomography
- EyeWiki (AAO), Optical Coherence Tomography: interferometry and low-coherence light; time-domain 10–15 µm, spectral-domain 20,000–40,000 A-scans/s and 3–5 µm, swept-source speeds; clinical uses; limits from media opacity and movement. https://eyewiki.org/Optical_Coherence_Tomography
- NICE, Glaucoma: diagnosis and management (NG81): optic nerve assessment with OCT or nerve head image where available; baseline imaging at diagnosis; OCT not a stand-alone tool. https://www.nice.org.uk/guidance/ng81
- NHS screening blog (26 Nov 2024), Using OCT eye scans in post-screening pathway will save tens of thousands of NHS hospital appointments every year: OCT in diabetic eye screening follow-up; cross-section of the retina identifies some treatable disease more accurately than a photograph. https://nationalscreening.blog.gov.uk/2024/11/26/using-oct-eye-scans-in-post-screening-pathway-will-save-tens-of-thousands-of-nhs-hospital-appointments-every-year/
- Optometry Scotland report, reported by Eye News UK (9 Oct 2025): OCT not funded through General Ophthalmic Services; cost the main barrier. https://www.eyenews.uk.com/news/post/new-report-calls-for-national-funding-to-expand-life-changing-eye-care-technology
- West Suffolk NHS Foundation Trust, Optical Coherence Tomography (OCT) patient leaflet 6555-1: cross-section images using light waves; dilating drops may be given beforehand. https://www.wsh.nhs.uk/CMS-Documents/Patient-leaflets/EyeTreatmentCentre/6555-1-Optical-Coherence-Tomography-OCT.pdf
- Royal Victoria Eye and Ear Hospital (Dublin), OCT Patient Info: light, not ionising radiation. https://www.rveeh.ie/app/uploads/2026/02/OCT-Patient-Info.pdf
- Roach L. Why Screen Multifocal IOL Patients With OCT. AAO EyeNet, 31 May 2020: OCT detects macular pathology in 9–30% of normal-appearing retinas; "a premium IOL requires a premium macula". https://aao.org/eyenet/article/why-screen-multifocal-iol-patients-with-oct
- Leung EH, Gibbons A, Koch DD. Cost-Effectiveness of Preoperative Optical Coherence Tomography in Cataract Evaluation for Multifocal Intraocular Lens. Ophthalmology 2020: about 20.5% of cataract patients have macular pathology, 11% possibly undetected clinically. https://pmc.ncbi.nlm.nih.gov/articles/PMC7311225
- Zimmermann H, Oberwahrenbrock T, Brandt AU, Paul F, Dörr J. Optical coherence tomography for retinal imaging in multiple sclerosis. Degener Neurol Neuromuscul Dis 2014: about 20 µm RNFL loss after optic neuritis; about 10 µm thinner in MS eyes without optic neuritis; OCT complementary to MRI, not yet standard for diagnosis. https://pmc.ncbi.nlm.nih.gov/articles/PMC7337265
- Shields CL, Manalac J, Das C, Saktanasate J, Shields JA. Review of spectral domain enhanced depth imaging optical coherence tomography of tumors of the choroid. Indian J Ophthalmol 2015;63(2): shaggy photoreceptors in 49% of melanomas and 0% of naevi; limits for peripheral tumours and lesions over 5 mm. https://pmc.ncbi.nlm.nih.gov/articles/PMC4399119
- Gella L, Raman R, Sharma T. Macular thickness measurements using Copernicus Spectral Domain Optical Coherence Tomography. Saudi J Ophthalmol 2014: central thickness 229–260 µm across devices; devices not interchangeable. https://pmc.ncbi.nlm.nih.gov/articles/PMC4398812
- Addis V, et al. Evaluation of the Cirrus HD-OCT Normative Database Probability Codes in a Black American Population. Ophthalmol Glaucoma 2021: colour-code definitions (green 5th–95th percentile, yellow 1st–5th, red at or below 1st); unexpected codes in healthy eyes. https://pmc.ncbi.nlm.nih.gov/articles/PMC8608902
- Poon LY-C, Wang C-H, Lin P-W, Wu P-C. The Prevalence of Optical Coherence Tomography Artifacts in High Myopia and its Influence on Glaucoma Diagnosis. J Glaucoma 2023: artefacts in 51.9% of high myopes vs 18.6% of others. https://pmc.ncbi.nlm.nih.gov/articles/PMC10453355/
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