Glaucoma tests explained: OCT vs visual field

If you’re being watched for glaucoma, you’ll meet two tests with confusing names: the OCT scan and the visual field test. Patients often ask why we do both, or whether one can replace the other. The honest answer is that they measure two different things, and glaucoma is only tracked properly when we follow both over time.

What glaucoma actually does

Glaucoma slowly damages the optic nerve, the cable that carries vision from the eye to the brain. It usually takes your side vision first, so gradually that you don’t notice until a lot is already gone. Because you can’t feel it happening, we don’t rely on how your eyes feel. We rely on measurement.

The OCT scan: structure

An OCT (optical coherence tomography) is a quick, painless scan that photographs the optic nerve and measures the thickness of the nerve-fibre layer around it. Think of it as measuring the structure, how many nerve fibres are still there.

• It’s fast and needs nothing from you but to sit still and look at a target.
• It can pick up thinning early, sometimes before you’d notice any vision change at all.
• We compare each scan to your baseline to see if the nerve is losing fibres over time.

The visual field test: function

A visual field test (perimetry) measures something different, your function: how much you can actually see across your field of vision. You look into a bowl and press a button each time you catch a faint flash of light at the edges.

• It’s not painful, but it needs concentration and can feel tiring, that’s normal.
• It maps where vision is already weak, especially the side vision glaucoma attacks first.
• Done repeatedly, it shows whether your usable vision is holding steady or slipping.

Why you need both

Structure and function don’t always change at the same time. An OCT can flag nerve thinning before a field test shows a gap, while a field test can confirm whether that thinning is actually costing you usable sight. One without the other is half the picture. Together, compared visit after visit, they let us catch change early and adjust treatment before you lose vision, not after.

The real secret: trends, not single readings

No single OCT or field test diagnoses glaucoma on its own, and one slightly off result rarely means disaster. What matters is the trend across visits. That’s why glaucoma care is a long relationship, and why we compare every scan to your last one rather than judging it in isolation. It’s also why keeping your follow-up appointments matters more than almost anything else you can do.

Being monitored for glaucoma, or have a family history of it? Read about how we track glaucoma or ask us on WhatsApp. A baseline OCT and field test give you a starting point to protect.