OCT Disc and Glaucoma Reference Card: RNFL, GCIPL and Pitfalls
Last updated: October 2026
A checklist for optic disc and ganglion cell OCT reports. The colour codes compare the patient with a normative database, so they help, but they can mislead in both directions.
Step 1: is the scan reliable?
- Signal strength above threshold, the circle centred on the disc, and no segmentation errors on the circular B-scan.
- Check disc area: very small or very large discs, tilted myopic discs and peripapillary atrophy fall outside most normative databases.
- Note the patient's age, ethnicity and refraction. Most databases do not include high myopes.
Step 2: the numbers
| Parameter | Typical normal | Notes |
|---|---|---|
| Average RNFL thickness | About 90–105 µm (falls by roughly 0.2–0.5 µm a year with age) | Values in the 70s or lower are suspicious; under about 50–55 µm the "floor effect" makes RNFL useless for tracking progression |
| Quadrant pattern (ISNT) | Inferior thickest, then superior, then nasal, then temporal | Loss of the inferior or superior "humps" on the TSNIT curve is an early glaucoma sign |
| Inter-eye asymmetry (average RNFL) | Under about 9 µm | Larger asymmetry is suspicious even when both eyes are "green" |
| Ganglion cell-inner plexiform layer (GCIPL, Cirrus) | Average about 80–85 µm | Inferotemporal arcuate thinning is typical of glaucoma. Useful in high myopes and for early central damage. |
| Rim parameters (BMO-MRW, Spectralis) | Compare with the device's age-matched database | BMO-based rim width is more anatomically accurate than the disc margin seen clinically |
Device note: RNFL values differ between Cirrus, Spectralis, Topcon and Optovue (typically by several microns). Use one device for follow-up, and never compare raw numbers across machines.
Step 3: "red disease" and "green disease"
| Trap | What happens | Examples |
|---|---|---|
| Red disease (false positive) | Red or yellow flags in an eye without glaucoma | High myopia and tilted discs (temporal shift of the RNFL bundles), small discs, segmentation errors, peripapillary atrophy, poor signal, ethnic variation |
| Green disease (false negative) | Normal colours despite real glaucoma | Large discs and high baseline RNFL, early focal notches averaged out, swelling (papilloedema, NAION) or ERM masking loss, vitreopapillary traction |
Step 4: patterns that point away from glaucoma
- Temporal (papillomacular) RNFL loss and central GCIPL loss: think of toxic or nutritional optic neuropathy, optic neuritis, Leber hereditary optic neuropathy and compressive lesions.
- Band or "bow-tie" loss (nasal and temporal): chiasmal or optic tract lesions; check the GCIPL for a hemianopic (vertical-midline) pattern.
- Thick RNFL: disc swelling (papilloedema, NAION, papillitis) or disc drusen. See drusen vs papilloedema.
- Pallor out of proportion to cupping, young age, or vision loss out of keeping with the field: image the brain.
Step 5: progression
- Use the device's guided progression analysis with at least 3–4 good-quality scans. A sustained loss larger than the test-retest variability (typically about 4–5 µm for average RNFL on spectral-domain OCT) suggests change.
- Rate of loss matters more than a single value: compare the slope with the patient's age and life expectancy (see target IOP).
- In advanced disease, follow GCIPL, macular OCT and visual fields rather than RNFL.
Never diagnose glaucoma from OCT colours alone. Combine the disc examination (rim, notches, disc haemorrhage), the field, the IOP and the OCT, and look for agreement between structure and function.
Related: how to read an OCT, disc damage likelihood scale, visual field card and disc haemorrhage.
Key sources
- Chong GT, Lee RK. Glaucoma versus red disease: imaging and glaucoma diagnosis. Curr Opin Ophthalmol 2012;23:79–88
- Mwanza JC et al. Ability of Cirrus HD-OCT optic nerve head parameters to discriminate normal from glaucomatous eyes. Ophthalmology 2011;118:241–8
- Chauhan BC et al. Bruch's membrane opening minimum rim width and RNFL for glaucoma detection. Ophthalmology 2015;122:1786–94
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