Visual fields: the jargon

OT  explores the terms commonly used on a visual field plot


1. Reliability indicators

These scores tell you how reliable the result of the test is. There are lots of things that can affect the reliability: distractions in the room, incorrect trial lens making the screen blurred, poor instructions or the patient doesn’t really understand what they are doing. A good perimetrist will control as many of these factors as possible. If you keep being asked to repeat field tests, have a conversation with the optometrist about things you might be able to do to improve the results. 

Fixation losses

The machine counts the times the patient loses their fixation and looks away from the fixation target. This tells you if the patient’s eye was wandering. If so, the result is less reliable.

Fixation monitor: ‘Blind spot’

The machine works out where the person’s natural blind spot is and occasionally flashes a spot directly in it. If the patient is looking at the central fixation target (usually a steady central yellow light) they shouldn’t be able to see the flash in their natural blind spot. If they do click the button, it was either a mistake or they have moved their eye off the fixation target.

False POS errors

A high score means the patient was ‘trigger happy’ clicking the button when there wasn’t a flash. They might be over-eager or just getting into a rhythm of clicking the button. This can also indicate poor understanding or lapsed concentration. Reinstruct the patient and try to calm them down.

False NEG errors

The patient doesn’t click despite the flash being one that the machine thinks they should have seen, for example, it was brighter than one they saw before. False negatives occur when the patient has lost concentration, but they can also happen in a genuine field defect so a result shouldn’t be disregarded just because of a high false negative score.

In conditions such as strokes, which affect the brain, the visual field is affected (mostly equally) in both eyes. In such cases, a defect would be seen on the same side of the visual field


2. Test Type

Stimulus: III White

The size (4mm2) and colour of the flashes. The flashes can be made larger if the patient has very poor sight.

Background: 31.5 asb

The brightness in ‘apostilbs’ of the screen background. Make sure you turn the machine on in the morning under conditions you normally use for the test


The program used to perform the field test. SITA STANDARD 24-2 takes about seven minutes per eye and goes into detail, testing 54 points in the visual field. SITA FAST 24-2 is quicker, at around four minutes per eye, and is more for screening than monitoring defects over time.

3. Patient data


The correct date of birth is vital when doing the test, not only to identify the patient but the machine sets the brightness of the flashes according to the patient’s age. Younger patients will have dimmer flashes shown to them than older patients, so an error in the date of birth will make the test too easy or too difficult and mess up the result.


The prescription of the reading glasses or trial lens used for the test. If this isn’t correct, the screen will be blurred, making the test result unreliable.

4. The results

GHT: glaucoma hemifield test

This is a text explanation of how likely the person is to have glaucoma. If the top half and bottom half of the field plot have different results, the plot will state: ‘Outside Normal Limits’ and the result is suspicious of disease.

VFI: visual field index

This gives a percentage score for how the patient performed compared to how they should do for their age. A lower percentage indicates visual field loss.

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