Scoring systems for measuring progression of visual field loss in clinical trials of glaucoma treatment

Objectives: To compare the statistical properties of two visual field scoring systems used by clinical trials of glaucoma treatments with a view to their performance as longitudinal measures of visual field progression. Design: Cohort study comparing the scoring systems used by the Advanced Glaucoma Intervention Study (AGIS) and the Collaborative Initial Glaucoma Treatment Study (CIGTS) using pairs of visual fields (C-30-2 of the Humphrey Analyzer) spaced 1 year apart. Participants: Sixty-seven eyes of 56 subjects with glaucomatous visual field loss from a natural history study of glaucoma were examined. Methods: Because both scoring systems are scaled so that they range from 0 to 20, the AGIS and CIGTS scores were compared by scatterplot, Pearson correlation, and the mean difference between scores. The difference between pairs of scores 1 year apart was used to compare the temporal variability of each scoring system. The proportion of subjects whose visual field 'deteriorated' in 1 year by the amount specified for unconfirmed progression in each clinical trial was calculated. Main Outcome Measures: Mean difference in scores 1 year apart and the variance of these differences were measured. Results: The median scores for the first of the two fields was 5.2 (interquartile range [IQR] = 9.8) for CIGTS and 5.0 (IQR = 7.0) for AGIS. The CIGTS scores were slightly larger than AGIS scores by an average of 0.5 (P = 0.06). The mean CIGTS score of the baseline fields was 0.06 (standard deviation = 3.8) higher than the mean of the second fields and 0.12 (standard deviation = 2.8) higher for AGIS. Fifteen percent of eyes had CIGTS scores that deteriorated by three or more (the cutoff for unconfirmed progression) over 1 year, while 7.5% of eyes had AGIS scores that deteriorated by four or more (the AGIS cutoff for unconfirmed progression) over the same period. Twenty-one percent improved by CIGTS criteria, while 12% improved by AGIS criteria. Conclusions: CIGTS scores are systematically slightly higher than AGIS scores. The CIGTS scoring system has higher temporal variability than the AGIS system. The CIGTS criterion for unconfirmed progression over a 1- year period is likely to lead to higher estimated rates of progression than the AGIS criteria. This is mostly because of the difference in the definition of progression between the two study criteria.