Objective: To compare the refractive results between open-field auto ref/keratometer and conventional autorefractor, and to investigate the effect of cycloplegic to this difference.
Methods: Three hundred and four primary and secondary school students were consecutively enrolled in Beijing Tongren Hospital. Non-cycloplegic and cycloplegic objective refractions were performed for each subject by conventional autorefractor (Accuref-K9001, Shin Nippon, Japan) and binocular, open-field auto ref/keratometer (Grand Seiko Co., Ltd., Hiroshima, Japan). The coincidence rate of sphere, spherical equivalent (SE) and axis (defined as difference of diopter ≤ 0.50 D, difference of axis degree ≤ 20°) were calculated; Bland-Altman and distribution analysis were performed according to mean and difference of SE.
Results: The coincidence rate of sphere, SE and axis were 77.3%, 78.6% and 66.0% before cycloplegic and increased to 94.4%, 95.1% and 69.5% after cycloplegic, respectively. The difference (95%CI) of SE before cycloplegic between these two refractometers was 0.12 (-1.04 to 1.29) D and was positively correlated with the mean of SE (after cycloplegic) (r(pearson) = 0.21, P < 0.001). The difference (95%CI) of SE after cycloplegic was -0.08 (-0.60 to 0.45) D. The difference (95%CI) of SE of K9001 autorefractor before and after cycloplegic was larger than that of WAM autorefractor [0.51 (-0.83 - 1.84) D and 0.31 (-0.66 to 1.28) D, P < 0.001]. Before cycloplegic, SE measured by WAM autorefractor showed myopic more than 0.25 D than K9001 (group 1) was found in 51 (16.8%) subjects; difference within 0.25 D was found in 160 (52.6%) subjects; hyperopic more than 0.25 D (group 3) was found in 93 (30.6%) subjects. After cycloplegic, 69 (22.7%) subjects were found in group 1, and subjects increased to 213 (70.1%) and decreased to 22 (7.2%) in group 2 and group 3, respectively.
Conclusion: The binocular, open-field auto ref/keratometer provides more hyperopic readings than conventional autorefractor. It will be useful in both clinical screening and scientific research because it produces less instrument myopia than that of conventional autorefractor.