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Assigning Utility Weights to Visual Field Deficits Using Methodological Assumptions

This study estimated the patient visual field defect in decibels (dBs) that corresponded with visual acuity values of 20/40, 20/70, 20/200 (US blindness), 20/400 (WHO blindness), and 20/1,000 (extreme impairment) using two methods. The first method categorized visual field defects by applying the logic of the Snellen acuity minimum angle of resolution (MAR) scale to the ability to perceive luminance adjusting for the differences in the acuity and luminance scales (MAR-based). The second method categorized visual field defects based on assumptions. We then assigned each visual field category the same qualtiy adjusted life (QALY) value associated with the analogous acuity category (Assumption-based).

The MAR-based method estimated that Mean Deviation (MDs) of –16, –18, –23, –24, and –30 or greater dBs corresponded to acuity values of 20/40, 20/70, 20/200, 20/400, and 20/1,000, respectively. For the same acuity values, the Assumption-based method resulted in MD estimates of –13, –17, –22, –24, and –30. Our MAR-based method yielded the function, QALYs = 0.98991 + 0.0022 . dBs – 0.00080518 . dBs2, setting dBs equal to the absolute value of the mean deviation in the better-seeing eye. Our Assumption-based method yielded the piece-wise function, QALYs = 1.0 – 0.0155 . dBs for dBs <22, and 1.815 – 0.0525 . dBs for dBs ≥ 22.

Relationship between Absolute Value of Mean Deviation from Normal in dBs and Quality of Life Losses Using three Possible Methodological Assumptions to Relate QALY Losses from Field Deficits to QALY Losses from Acuity Impairment.

This study estimated the patient visual field defect in decibels (dBs) that corresponded with visual acuity values of 20/40, 20/70, 20/200 (US blindness), 20/400 (WHO blindness), and 20/1,000 (extreme impairment) using two methods. Using the logic of the Snellen acuity minimum angle of resolution (MAR) and categorizing visual field defects based on assumptions, we assigned each visual field category the same QALY value associated with the analogous acuity category (Assumption-based). The MAR-based method estimated that MDs of –16, –18, –23, –24, and –30 or greater dBs corresponded to acuity values of 20/40, 20/70, 20/200, 20/400, and 20/1,000, respectively. For the same acuity values, the Assumption-based method resulted in MD estimates of –13, –17, –22, –24, and –30. Our MAR-based method yielded the function, QALYs = 0.98991 + 0.0022 . dBs – 0.00080518 . dBs2, setting dBs equal to the absolute value of the mean deviation in the better-seeing eye. Our Assumption-based method yielded the piece-wise function, QALYs = 1.0 – 0.0155 . dBs for dBs <22, and 1.815 – 0.0525 . dBs for dBs ≥ 22.

 

MAR refers to the equation to assign QALY losses to visual field deficits that results from assigning visual field deficit categories to their analogous acuity loss categories based on the minimum angle of resolution methodology. Assumption-PW and Assumption-Poly refer to the point-wise linear equation and the polynomial equation used to assign QALY losses to visual field loss based on assumptions regarding which visual field loss values are analogous to acuity values of 20/20, 20/200, and complete impairment.
 

Rein DB, Wirth KE, Johnson CA, Lee PP. Estimating quality-adjusted life year losses associated with visual field deficits using methodological approaches. Ophthalmic Epidemiol 2007;14(4):258–264.
 

 
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