Frequency of testing for detecting visual field progression
- Correspondence to: Dr D P Crabb, Faculty of Science and Mathematics, Nottingham Trent University, Clifton Campus, Nottingham NG11 8NS, UK; david.crabb{at}ntu.ac.uk
- Accepted 30 January 2002
Abstract
Aims: To investigate the effect of frequency of testing on the determination of visual field progression using pointwise linear regression (PLR).
Methods: A “virtual eye” was developed to simulate series of sensitivities over time at a given point in the eye. The user can input the actual behaviour of the point (for example, stable or deteriorating steadily), and then a configurable amount of noise is added to produce a realistic series over time. The advantage of this over using patient data is that the actual status of the eye is known. Series were generated using different frequencies of testing, and the diagnosis that would have been made from each series was compared with the true status of the eye. A point was diagnosed as progressing if the regression line for the series showed a deterioration of at least 1 dB per year, significant at the 1% level. From these results, graphs were produced showing the number of points correctly or incorrectly diagnosed as progressing.
Results: With the virtual eye deteriorating at a rate of 2 dB/year, it was found that the point was determined to be progressing quicker when more tests were carried out each year. With a stable virtual eye, it was found that increasing the frequency of testing increased the number of series that were falsely labelled as progressing during the first 3 years of testing.
Conclusions: As the frequency of testing increases, the sensitivity of PLR increases. However, the specificity decreases; possibly meaning more unnecessary changes in treatment. Three tests per year provide a good compromise between sensitivity and specificity.
