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Shrikant Bharadwaj, Vinay Kumar, Geetha Sravani; Racial difference in the defocus calibration of eccentric infrared photorefraction. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2724.
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Refraction estimates obtained using eccentric infrared (IR) photorefraction depend critically on the calibration of luminance slope across the pupil into diopters (defocus calibration). While the inter- and intra-subject variability of defocus calibration is well known, there is little information on its variability across people of different races. This study determined the race-dependent variability in defocus calibration of the PlusOptix PowerRef3®, a commercially available IR photorefractor, and explored the impact of calibration factor on refractive error estimates obtained using this instrument.
In Experiment I, 30 subjects each of Indian, African, East Asian and European descent (mean±1SD age: 26.3±7.8yrs) fixated on the photorefractor at 1m with their right eye while trial lenses from +5D to -5D were placed in 1D steps before their IR filter occluded left eye. The anisometropia recorded by the photorefractor was plotted against the induced lens power to obtain the defocus calibration factor. In Experiment II, refractive error estimates of 50 myopic Indian eyes (24.5±3.5yrs) obtained from the instrument using its inbuilt calibration factor or by scaling them using the race-specific population average or the individual’s own calibration factor was compared against the corresponding gold-standard retinoscopy values.
Median calibration factors were 1.56 (IQR: 1.41 - 1.73), 1.45 (1.25 - 1.68), 1.38 (1.19 - 1.70) and 1.17 (1.06 - 1.31) for Indians, Africans, East Asians and Europeans, respectively, relative to the instrument’s inbuilt calibration value. The calibration factors of Indians, Africans and East Asians were significantly larger than those of Europeans (p≤0.005) while they were not different from each other (p=0.53). Relative to retinoscopy, myopia was over-estimated by 64% when using the instrument’s inbuilt calibration factor and by only 6% and 7% when using the race-specific population average and the individual’s own calibration factor, respectively.
Defocus calibration of IR photorefraction shows race dependent variability, in addition to the within-race inter-subject variability. Using the calibration factor of one race might therefore lead to multiplicative errors in refraction estimates of other races. These errors can be minimized by using a race-specific population average calibration factor or using the individual’s own calibration factor.
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