Resolving the clinical acuity categories hand motion and counting fingers using the Freiburg Visual Acuity Test (FrACT)

Purpose The Freiburg Visual Acuity Test (FrACT) has been suggested as a promising test for quantifying the visual acuity (VA) of patients with very low vision, a condition often classified using the semi-quantitative clinical scale counting fingers (CF), hand motion (HM), light perception (LP) and no light perception. The present study was designed to assess FrACT performance in a sizable number of CF, HM, and LP patients in order to generate a setting for future clinical studies in the low vision range. Methods We examined a total of 41 patients ( LP, n=11; CF, n=15; HM, n=15) with various eye diseases ( e. g., diabetic retinopathy, ARMD), covering the clinical VA scale from LP to CF. The FrACT optotypes were presented at a distance of 50 cm on a 17-inch LCD monitor with four random orientations. After training, two FrACT measurements ( test and retest) were taken, each comprising 30 trials. Results FrACT measures reproducibly the VA of CF and HM patients. In CF patients, FrACT resulted in a mean logMAR=1.98 +/- 0.24 (corresponding to a decimal VA of 0.010), for HM in a mean logMAR=2.28 +/- 0.15 (corresponding to a decimal VA of 0.0052). In all LP patients the FrACT values were close to what would be obtained by random guessing. The mean test-retest 95% confidence interval was 0.21 logMAR for CF patients and 0.31 logMAR for HM respectively. Test-retest variability declined from 24 to 30 trials, showing that at least 30 trials are necessary. Conclusion FrACT can reproducibly quantify VA in the CF and HM range. We observed a floor effect for LP, and it was not quantifiable further. Quantitative VA measures are thus obtainable in the very low-vision range using FrACT.