Visual field progression in glaucoma: What is the specificity of the guided progression analysis?

Purpose: To estimate the specificity of the Guided Progression Analysis (GPA) (Carl Zeiss Meditec, Dublin, CA) in individual patients with glaucoma.

Design: Observational cohort study.

Participants: Thirty patients with open-angle glaucoma.

Methods: In 30 patients with open-angle glaucoma, 1 eye (median mean deviation [MD], -2.5 decibels [dB]; interquartile range, -4.4 to -1.3 dB) was tested 12 times over 3 months (Humphrey Field Analyzer, Carl Zeiss Meditec; SITA Standard, 24-2). "Possible progression" and "likely progression" were determined with the GPA. These analyses were repeated after the order of the tests had been randomly rearranged (1000 unique permutations).

Main Outcome Measures: Rate of false-positive alerts of "possible progression" and "likely progression" with the GPA.

Results: On average, the specificity of the GPA "likely progression" alert was high - for the entire sample, the mean rate of false-positive alerts after 10 follow-up tests was 2.6%. With "possible progression," the specificity was considerably lower (false-positive rate, 18.5%). Most important, the cumulative rate of false-positive alerts varied substantially among patients, from <1% to 80% with "possible progression" and from <0.1% to 20% with "likely progression." Factors associated with false-positive alerts were visual field variability (standard deviation of MD, Spearman's rho = 0.41, P<0.001) and the reliability indices (proportion of false-positive and false-negative responses, fixation losses, rho>0.31, P≤0.10).

Conclusions: On average, progression criteria currently used in the GPA have high specificity, but some patients are more likely to show false-positive alerts than others. This is a natural consequence of populationbased change criteria and may not matter in clinical trials and studies in which large groups of patients are compared. However, it must be considered when the GPA is used in clinical practice where specificity needs to be controlled for individual patients.