# OCT Interpretation

> OCT interpretation for Optometry: macular thickness maps, RNFL norms, normative database traps, artefacts and correlating scans with discs and fields.

- Canonical URL: https://prepelephant.com/topics/allied/optometry/oct-interpretation-optometry
- Exam / course: Allied Health · Subject: Optometry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "OCT Interpretation", PrepElephant, https://prepelephant.com/topics/allied/optometry/oct-interpretation-optometry

## Direct answer

Optical coherence tomography is ultrasound's optical cousin: low-coherence interferometry times the echoes of infrared light back-scattered from the retinal layers, resolving detail of a few micrometres — cross-sectional structure in vivo, quantified. Reading a report follows a fixed drill: check signal strength and centration, then the map numbers against device norms (central macular thickness roughly 250-280 micrometres depending on platform; average retinal nerve fibre layer about 90-100 micrometres), then the normative colours (green within the database, yellow borderline, red below the first percentile), then inter-eye symmetry, and finally correlation with the disc appearance and visual field. The colours are an argument, not a verdict — high myopes flag red falsely ("red disease") and early glaucoma hides inside green ("green disease") — which is why the optometrist interprets the OCT, never the reverse.

## What you must remember

- **Physics in a line:** near-infrared low-coherence light, echo delay encoded by interferometry, axial resolution of a few micrometres; spectral-domain dominates, swept-source at longer wavelengths images choroid better.
- **Macular map:** ETDRS nine-sector grid with the central 1 mm subfield — device-dependent normal central thickness roughly 250-280 micrometres; know your machine's normative database and never compare across platforms.
- **RNFL norms:** average thickness about 90-100 micrometres in healthy adults, thinning with age; the quadrant clock-hour map and ganglion cell complex analyses refine glaucoma detection, respecting the ISNT pattern of healthy nerves.
- **Reading order that never varies:** image quality (signal strength, centration, segmentation lines) before numbers, numbers before colours, colours before conclusions; inter-eye asymmetry exceeding roughly 5-10 micrometres of RNFL is meaningful even when both eyes sit "green".
- **Red and green disease:** normative databases skew to older, non-myopic eyes — high myopes and large discs flag falsely red; early glaucomatous thinning can sit within green — disc appearance and fields arbitrate.
- **Artefact catalogue:** blink and motion lines, decentration, epiretinal membrane or large vessels dragging segmentation, shadowing from vitreous floaters and dense cataract dropping signal strength below acceptance.
- **Pathology signatures:** subretinal fluid in central serous chorioretinopathy, intraretinal cysts in macular oedema, drusen and pigment epithelial detachments in age-related degeneration, full-thickness defect in macular hole, taut posterior hyaloid in vitreomacular traction, and focal RNFL notches in glaucoma.
- **Progression discipline:** compare like scans with like (same protocol, same machine), guided progression analysis beats single-scan intuition, and structural change must march in step with functional change before management escalates.

## Reading an OCT report without the colours deciding

A 58-year-old glaucoma suspect: pressure 26 mmHg right, disc excavated inferiorly. The OCT shows signal strength 9, good centration, segmentation hugging the right layers — trustworthy. Average RNFL 68 micrometres, red on the normative chart, the 7 o'clock hour at 42; the ganglion cell map shows matching inferior-temporal loss. The left eye averages 92 micrometres, green throughout. Asymmetry of 24 micrometres dwarfs any ageing explanation; the 24-2 field confirms a superior nasal step. Conclusion: glaucomatous damage, right eye — the colours merely decorate a conclusion the numbers and symmetry had already drawn.

Now the counter-case: a 26-year-old high myope at −9.00 D flags red diffusely on RNFL, average 78 micrometres, healthy discs, full fields, normal pressure. The database simply lacks many −9 D eyes whose stretched retinas lay their fibres thin; this is red disease. Declaring her a glaucoma patient on colour alone starts a decade of unnecessary drops and anxiety. The optometrist's value is precisely this adjudication — scan quality, then numbers, then the eye they came from.

## Where students slip

Quoting universal cut-offs: there is no "normal central thickness" independent of machine — platforms differ systematically and are never interchangeable. Reading the summary colour first, ignoring segmentation errors riding under a clean-looking printout, and skipping artefact interrogation — a decentred macular cube mislabels sectors, and a low-signal scan through cataract thins RNFL spuriously. Treating OCT as diagnostic of glaucoma alone: macular cysts, holes and diabetic oedema referrals are optometric bread and butter, and central thickness thresholds drive anti-VEGF referral letters. And the classic examination answer: OCT is a structural test; function still needs a field, and the pair must agree before the diagnosis does.

## Frequently asked questions

### What normal central macular thickness should be quoted?

Roughly 250-280 micrometres for the central subfield — but strictly the device's own normative value, since platforms differ systematically and are never interchangeable.

### What is red disease on OCT?

Falsely "abnormal" colour coding in eyes outside the normative database — typically high myopes or large discs — read as glaucoma despite healthy discs and fields.

### Which quality checks precede interpretation?

Signal strength within acceptance, correct centration, intact segmentation lines, and absence of blink or motion artefact — before any number or colour is believed.

### Which OCT signs indicate macular oedema versus central serous chorioretinopathy?

Intraretinal cystic spaces for oedema versus subretinal fluid beneath a detached neurosensory retina in central serous — different spaces, different referrals.

### Why must RNFL asymmetry be checked even with both eyes green?

Inter-eye differences beyond roughly 5-10 micrometres suggest early glaucomatous loss that each eye's within-normal comparison conceals — symmetry is the early-warning channel.