| Sumario: | Background: The majority of the visually impaired population lives in rural areas, where, at times, it is difficult to obtain consistent visual acuity (VA) chart luminance levels. Electronic devices, being self-luminous, would help overcome this barrier. The current study identified and validated a smartphone-based Snellen VA chart against the standard ETDRS chart for screening and clinical practice. Methods: The study was conducted in four phases. Phase I: VA chart apps on the Google Play store were explored, screened, reviewed, and calibrated based on the displayed optotype size. Phase II: photometric analysis of room illuminance and VA chart luminance levels was performed. Phase III: the selected smartphone-based Snellen VA chart app was validated against the standard ETDRS chart. Phase IV: repeatability of both the uncorrected and best-corrected VA was checked with the ETDRS chart and the smartphone-based Snellen VA chart app. Results: Phase I: The Snellen Chart app was found to display the Snellen optotype size accurately (inaccuracy <0.25mm). Phase II: average illuminance of the room was ~420 lux. The luminance level of the retro-illuminated ETDRS chart and the Android smartphone screen was ~180 cd/m2 and ~200 cd/ m2, respectively. Phase III: there were no statistically significant differences between the ETDRS chart and the Snellen Chart app for uncorrected (median difference: -0.02 logMAR, p=0.39) and best-corrected (median difference: 0.00 logMAR, p=0.94) VAs in all participants (n=200; p<0.05). The Snellen Chart app showed good agreement with the ETDRS chart for both uncorrected (limits of agreement: -0.16 to 0.22 logMAR) and bestcorrected (limits of agreement: -0.01 to 0.01 logMAR) VAs in all participants. Phase IV: Both the ETDRS chart and the Snellen Chart app were found to have good test-retest repeatability in participants with emmetropia (n=63) and myopia (n=34) with an overall coefficient of repeatability ranging between 0.03 and 0.05 logMAR (p≤0.10). Conclusions: VAs measured with the smartphonebased Snellen Chart app are reliable and in good agreement with the ETDRS chart. Thus, both VA assessment methods can be used interchangeably.
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