Last month we looked at how to test visual fields. This month, we will look at some common errors encountered.
Visual field testing is a subjective test and relies on accurate patient responses, and so this means errors can potentially creep in. Difficult-to-interpret results can be due to the patient not understanding the requirements for the test or maybe finding it difficult to respond. Some commonly encountered errors include false negatives, false positives, and fixation losses.
False negatives
False negatives occur when the patient does not press the button when a light is seen – but had pressed the button when the light was seen in the same location previously in the test. The most likely explanation for this is that the patient either lost concentration, became tired or maybe blinked at that moment, and is not uncommon.
If the false negatives go over 30 per cent, the test is considered unreliable and should be repeated. It is best to repeat the test on a different day as fatigue could be the reason. Also consider the time of day the test is taken, as the patient may become tired more quickly in the late afternoon compared to the morning.
When repeating the test, ensure the patient fully understands the requirements of the test, and perhaps allow the patient a short practise with the machine in test mode.
False positives
False positives occur when the patient presses the button when there has been no light presented. If more than 15 per cent of false positive are seen, then the test is considered unreliable and should be repeated.
Fixation losses
Fixation losses occur when the patient loses fixation and their eye wanders away from the central dot, which then causes the light stimulus to appear in their blind spot. The blind spot is where the optic nerve enters the eye, and there are no rods or cones present, hence the blind spot. If this happens, the machine will beep to inform you that the patient has lost fixation.
It is important to be present in the room whilst the patient takes the visual fields test, so that any problems encountered can be monitored, and you can provide encouragement to the patient; for example, reminding the patient to keep looking at the central dot and giving them time checks, although taking care not to interrupt their concentration.
It is also a good idea to practise the test yourself, as you may find that it is not that easy undertaking the test due to the high level of concentration needed, and an ability to perform the required tasks quickly. It can be helpful to realise how it is possible to make mistakes, and the degree of patience often needed when explaining the test to patients.
To conclude, it can be seen that the optical assistant plays an important part in visual field screening and can help to minimise any errors by careful explanation and understanding.
Sue Deal FBDO R is a practising dispensing optician, ABDO College examiner, senior tutor and supervisor for dispensing opticians. She is also a practice visitor and external moderator for ABDO. She was recently awarded the ABDO Medal of Excellence for her outstanding services to the profession.