OCT is a test done using the Stratus OCT instrument to measure the thickness of the retinal nerve fibre layer, as well as providing detailed microscopic maps of the retina and optic nerve at the back of the eye. It is very useful for the diagnosis and monitoring of a group of diseases, including glaucoma, diabetic retinopathy, age-related macular degeneration and retinal vein occlusions. The test is quick and painless, a little like having a photo taken, and when necessary can be repeated for monitoring.