A multilevel refraction protocol based on visual acuity

Abstract

Objective: To describe the historical development and methodology of a standard subjective refraction protocol to obtain best-corrected visual acuities in multicentre ophthalmic clinical trials. Methods: A revised, multilevel, subjective refraction protocol was developed for the Macular Photocoagulation Study (MPS) to accommodate poorer levels of vision than had previously been encountered. Results: Since its development in 1983, the multilevel refraction protocol has been adopted by several major clinical trials, is a reliable method to obtain best-corrected visual acuity among patients with a wide range of visual acuities and eye disorders, and has been shown to be more reliable than autorefraction. Among 82 participants in the MPS, 89% of repeated refractions obtained using the multilevel refraction protocol differed from each other by less than +/– 0.50 dioptre (D) and 97.5% by less than +/–1.00 D. Among 29 participants in the Submacular Surgery Trials (SST), the multilevel refraction protocol gave a spherical equivalent that averaged 1.04 D more positive than autorefraction (95% limits of agreement: 0.74, 1.34), and the visual acuity score, using the refraction protocol, was 1.5 letters better compared with autorefraction (95% limits of agreement: 0.0, 3.0). Conclusions: The multilevel refraction protocol is a reliable method of obtaining best-corrected visual acuity among patients across a wide range of visual acuities and many different eye disorders in ophthalmic clinical trials.