Purpose: We used multi-attribute theory to develop a utility elicitation instrument for people with glaucoma based on the National Eye Institute-Visual Functioning Questionnaire (NEI-VFQ), a vision specific quality of life instrument frequently used in clinical trials. We evaluated the construct validity of the instrument by comparing estimated utility values to glaucoma severity as measured by mean deviation (MD).
Methods: NEI-VFQ responses from 99 patients with a range of glaucoma severity were analyzed qualitatively to identify the 12 items most responsive to visual field status. We then constructed a conjoint interview and administered it to 48 people with glaucoma, identifying the 6 items of greatest importance. With these results, we constructed a web-based interview using standard gamble (SG) and visual analog scales (VAS) and administered it to a community sample of 404 people. SG results were analyzed using regression methods and the VAS results were used to weight intermediate responses. We assessed validity by applying the weights to the NEI-VFQ responses of 709 participants and plotting these against MD. The instrument would be considered valid if greater disutility was associated with worse MD. Result: Disutility for people with glaucoma was determined to be measured by three items, with maximum utility loss shown in parentheses: being able to read normal print (0.11); having to stay home due to vision (0.125); and needing help with activities due to vision (0.036). The results of our construct validity analyses are presented in the accompanying table. As expected, utility loss was associated with increasing severity of disease in the best seeing-eye, indicating excellent construct validity. Conclusion: We have developed and validated an instrument that can be used to estimate the utility loss in people with glaucoma based upon the NEI-VFQ. When used in clinical trials, the instrument will provide an estimate of the utility loss associated with the progression of glaucoma, supporting cost-utility studies of glaucoma progression and preventions. 
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