Reliability of visual acuity measurements and screening under Field conditions

Purpose: The purpose of this analysis was to investigate the reliability of visual acuity screening and measurements made under field conditions when standardization of all test conditions is not possible. Setting and Methods: In the Washington County Follow-Up Eye Study conducted in 1985 among cases of ocular histoplasmosis and controls from the same community who had been selected in 1970, a primary goal was to obtain a current visual acuity measurement of each eye of each participant in order to assess 15-year changes in visual acuity. Both visual acuity screening and measurement were accomplished on two separate occasions and in two separate locations (home or workplace and clinic) for 308 eyes and 317 eyes, respectively, of 161 participants. Screening was used to classify the visual acuity as 20/40 or better, worse than 20/40 but at least 20/160, or worse than 20/160. Screening and measurement were accomplished on at least one occasion for the eyes of 192 of 216 eligible individuals. Results: Among 308 eyes screened twice, 289 (94%were classified identically (k̂equals;=0.80). Among 317 eyes with visual acuity measured on two occasions, the measurements were within two lines for 305 eyes (96%; k̂ = 0.50). Among eyes for which classifications of visual acuity from screening and measurement could be compared, 325 (96%of 340 assessed at home and 326 (96%of 338 assessed in clinic were classified in the same way (k̂ = 0.86 and 0.88, respectively). Conclusion: Despite the unavoidable lack of standardization of test conditions when visual acuity was screened and measured in the home or workplace, the results were highly reliable when compared with visual acuity testing in the clinic under standard conditions.