Converting visual acuity to utilities

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Abstract

Background: Utility theory can be used to quantify dysfunction associated with various diseases and thus can represent a “hard” measure of quality of life. By determining utility values, one can compare the quality of life of patients with ocular disease to that of patients with non-ophthalmic problems. We performed a study to determine whether utility values from patients with ocular disease are associated with clinical variables, including visual acuity in the better-seeing eye, and to develop a mathematical method for converting visual acuity to utility value, if there is an association between the two.

Methods: Cross-sectional study. A total of 239 patients from a tertiary care retinal practice with various ocular conditions, including macular degeneration, cataract, glaucoma and diabetic retinopathy, were interviewed under standardized conditions to determine their utility values by the time trade-off technique. Visual acuity, duration of visual loss and number of concomitant conditions were also determined. Multiple linear regression was performed to determine which variables were associated with utility values.

Results: The mean acuity in the better-seeing eye was 0.479 (near 20/40 vision). The mean utility value was 0.72. Accordingly, the average patient in our series was willing to trade 2.8 of every 10 remaining years of life to obtain perfect vision in both eyes. Utility value was significantly associated with visual acuity in the betterseeing eye (F = 69.1, p < 0.001). Other variables were not significantly associated with utility value. The association with duration of visual loss approached statistical significance (p = 0.075). Utility values (U) for patients with ocular disease can be derived from the following formula: U = (0.374)(visual acuity in better-seeing eye) + 0.514.

Interpretation: Utility values from patients with ocular disease were strongly associated with visual acuity and could be estimated mathematically.

Résumé

Contexte: La théorie de la valeur utilitaire peut servir à quantifier le dysfonctionnement associé à diverses maladies et offrir une mesure «reelle» de la qualité devie. En établissant les valeurs utilitaires, on peut comparer la qualité de vie des patients ayant une maladie oculaire à celle de ceux qui ont d'autres problèmes. Notre étude avait pour objet de déterminer si les valeurs utilitaires des patients ayant une maladie oculaire étaient associées à des variables cliniques, y compris l'acuité visuelle du meilleur œil, et, le cas échéant, de mettre au point une méthode mathématique pour convertir l'acuité visuelle en valeur utilitaire.

Méthodes: Enquête transversale. Nous avons interrogé 239 patients d'un cabinet de soins tertiaires de la rétine ayant diverses maladies oculaires, notamment : dégénérescence maculaire, cataracte, glaucome et rTtinopathie diabétique. Les entrevues, menées de façon standard, avaient pour objet d'établir leurs valeurs utilitaires par le biais de la technique du «troc temporel» (time trade-off). Nous avons aussi mesuré l'acuité visuelle, la durée de la perte visuelle et le nombre de conditions concomitantes. Nous avons effectué une régression linéaire multiple pour établir les variables associées aux valeurs utilitaires.

Résultats: L'acuité moyenne du meilleur œil etait de 0,479 (près d'une vision de 20/40), et la valeur utilitaire moyenne, de 0,72. En conséquence, le patient moyen de notre groupe acceptait de troquer 2,8 années de chaque décennie lui restant à vivre pour obtenir une vision parfaite des deux yeux. La valeur utilitaire était associée de façon importante à l'acuité visuelle chez l'œil ayant la meilleure vision (F = 69, I, p < 0,001). Les autres variables ne furent pas associees de façon importante à la valeur utilitaire. L'association avec la durée de la perte visuelle faisait presque du sens sur le plan statistique (p = 0,075). On peut déduire les valeurs utilitaires (U) des patients ayant une maladie oculaire selon la formule suivante : U = (0,374)(acuité visuelle de l'œil voyant le mieux) + 0,514.

Interprétation: Les valeurs utilitaires des patients ayant une maladie oculaire étaient associées de façon significative à l'acuite visuelle et on peut les estimer de façon mathématique.

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The Cost-Effective Ocular Health Policy Unit, Hotel Dieu Hospital, Queen's University, Kingston, Ont.

Evidence-Based Healthcare, Flourtown, Pa.

The Retina Vascular Unit, Wills Eye Hospital, Jefferson Medical College, Philadelphia, Pa.

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The School of Public Health, Harvard University, Boston, Mass.

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