Reliability indices help optometrists and ophthalmologists measure how accurate VF test results are. There are 4 reliability indices:
- The fixation loss error measures how good patients are at keeping their eyes fixed straight ahead, because if they move their 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 for patients to reduce their fixation loss error down to near zero.
- False positive errors occur when patients are too ’trigger happy’. It is only natural for patients to want to do well at the test and try hard to catch all the lights. The way to help patients avoid these types of errors is ask them to only press the button when they are sure they see a light. The visual field machine is trying to work out the exact threshold (lowest limit) of their vision, and therefore will present lights in locations at different brightness just below and just above what they can actually see. Therefore, even if there vision is entirely normal, it will still be normal for them not to see some of the lights they are being presented because they are below their threshold. Patients with visual field defects often develop more false positives, because of not being sure if they are seeing lights on the edge of the defect just out of view. A false positive error rate above 15% means it is a good idea to repeat the test and restate the above advice to reduce ‘trigger happy’ behaviour.
- False negative errors are effectively lights that have been presented to patients again in an area that they have already seen, and then for some reason they 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%, it is usually a good idea to 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 the patients has 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.