How do you know if your visual field tests are accurate?
Reliability indices help your OcuPlan optometrist and ophthalmologist measure how accurate your VF test results are. There are 4 reliability indices that they use:
- The fixation loss error measures how good you are at keeping your eyes fixed straight ahead, because if you move your eyes around to follow the lights it could invalidate the test results. Ideally we aim to keep fixation losses less than 10% to make the test accurate, but it would only need repeating if the fixation losses are greater than 20%. With more practice and discipline, it is possible to reduce your fixation loss error down to near zero.
- False positive errors occur when patients are too ’trigger happy’. It is only natural to want to do well at the test and try hard to catch all the lights, but the way to avoid these errors is to only press the button when you are sure you see a light. The visual field machine is trying to work out the exact threshold (lowest limit) of your vision, and therefore will present lights in locations at different brightness just below and just above what you can actually see. Therefore, even if your vision is entirely normal, it will still be normal not to see some of the lights you are being presented because they are below your threshold.
- False negative errors are effectively lights that have been presented to you again in an area that you have already seen, and then for some reason you miss them the second time around. It can sometimes be a sign of poor concentration, tiredness during the test or a fixation issue. If the false negative rate is above 15%, clinicians will often choose to repeat the test at another time.
- Finally, the time it takes to perform the test from the start of the programme to the end can give the clinician a good idea as to how well you have performed and how accurate the test has been. If the machine detects false positive and negative errors or fixation losses during the test, it has to repeat checks over these visual areas, and this takes more time. Also, if a patient become tired or loses concentration, the test takes longer and will be less accurate.
Your OcuPlan consultant will use all the above reliability indices to check your VF test is accurate and can explain these to you in clinic when they review your results.