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 both are done, or whether one can replace the other. The answer is that they measure two different things, and glaucoma is tracked by following both over time.

What glaucoma 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, eye doctors do not rely on how your eyes feel. They rely on measurement.

The OCT scan: structure

An OCT (optical coherence tomography) is a quick, non-contact scan that images 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.
• Each scan is compared 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 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 is normal.
• It maps where vision is already weak, especially the side vision glaucoma affects 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 costing you usable sight. One without the other is half the picture. Together, compared visit after visit, they help show change early, so that treatment can be adjusted.

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 is why glaucoma care is long-term, and why every scan is compared to your last one and not judged in isolation. It is also why keeping your follow-up appointments matters so much.

If glaucoma runs in your family, ask an eye doctor whether you need baseline tests.