Psychometrics of a new questionnaire to assess glaucoma adherence: the Glaucoma Treatment Compliance Assessment Tool (an American Ophthalmological Society thesis)

Abstract

Purpose: To report the psychometrics of the Glaucoma Treatment Compliance Assessment Tool (GTCAT), a new questionnaire designed to assess adherence with glaucoma therapy.

Methods: We developed the questionnaire according to the constructs of the Health Belief Model. We evaluated the questionnaire using data from a cross-sectional study with focus groups (n = 20) and a prospective observational case series (n=58). Principal components analysis provided assessment of construct validity. We repeated the questionnaire after 3 months for test-retest reliability. We evaluated predictive validity using an electronic dosing monitor as an objective measure of adherence.

Results: Focus group participants provided 931 statements related to adherence, of which 88.7% (826/931) could be categorized into the constructs of the Health Belief Model. Perceived barriers accounted for 31% (288/931) of statements, cues-to-action 14% (131/931), susceptibility 12% (116/931), benefits 12% (115/931), severity 10% (91/931), and self-efficacy 9% (85/931). The principal components analysis explained 77% of the variance with five components representing Health Belief Model constructs. Reliability analyses showed acceptable Cronbach's alphas (>.70) for four of the seven components (severity, susceptibility, barriers [eye drop administration], and barriers [discomfort]). Predictive validity was high, with several Health Belief Model questions significantly associated (P <.05) with adherence and a correlation coefficient (R (2)) of .40. Test-retest reliability was 90%.

Conclusion: The GTCAT shows excellent repeatability, content, construct, and predictive validity for glaucoma adherence. A multisite trial is needed to determine whether the results can be generalized and whether the questionnaire accurately measures the effect of interventions to increase adherence.

FIGURE 1

Number of focus group statements related to glaucoma adherence for each construct of the Health Belief Model. There were a total of 931 statements (n=20 participants). Table 1 describes the operational definitions for the Health Belief Model constructs. Glaucoma Treatment Compliance Assessment Tool Study.

FIGURE 2

Percentage of unique statements related to glaucoma adherence for each focus group from contextual analysis of focus group transcripts (n=20 participants). The percentage of unique statements dropped to less than 10% of statements during the 5th focus group indicating that the statements became redundant and no further focus groups are needed. Glaucoma Treatment Compliance Assessment Tool Study..

FIGURE 3

Mean Likert scores for the 21 Likert scale questions at Time 1 (initial visit) and Time 2 (3-month visit) in 35 participants with glaucoma or ocular hypertension. The asterisks(*) indicate that the Spearman’s rho shows statistically significant nonparametric correlation (P≤.05) between Time 1 and Time 2 for all questions except Q18 and Q26. Glaucoma Treatment Compliance Assessment Tool Study.

FIGURE 4

Bland-Altman analysis74 of the total score at Time 1 (initial visit) and Time 2 (3-month visit) in 35 participants with glaucoma or ocular hypertension. We created a total score of the Glaucoma Treatment Compliance Assessment Tool (GTCAT) at each time period by simple addition of the 21 Likert scale questions. It shows a fixed bias defined as the mean (± SD) difference (Time 1 – Time 2) between scores of .41 (±15.5) and no sign of proportional bias with higher or lower scores. No participant was an outlier as defined as a difference in scores outside the 95% confidence interval (dotted lines). Glaucoma Treatment Compliance Assessment Tool Study.