Repeatability and validity of astigmatism measurements

BACKGROUND: As more patients inquire about refractive surgical procedures, the measurement of astigmatism prior to surgery becomes more important in a ssessing refractive outcome. Knowledge of the repeatability of the astigmat ism measurement allows one to distinguish a true change in cylinder power f rom measurement error. METHODS: Forty adults with structurally normal eyes and refractive errors w ere evaluated for the repeatability of astigmatic refractive error measures . Noncycloplegic and cycloplegic measurements of refractive astigmatism wer e made by retinoscopy, subjective refraction, and autorefraction. All measu res were made at 2 visits within 2 weeks by the same examiner. Difference v ersus mean plots and the 95% limits of agreement of each technique determin ed the repeatability of a measurement and the agreement between the methods of measurement. RESULTS: The most reliable measure of astigmatic refractive error was cyclo plegic autorefraction, with 95% limits of agreement of +/-0.28 D, followed by noncycloplegic autorefraction (+/-0.35 D) and cycloplegic subjective ref raction (+/-0.44 D). Noncycloplegic retinoscopy was the least reliable asti gmatic refractive error measure, with interoccasion 95% limits of agreement of +/-1.02 D. The most repeatable measurement of cylinder axis was cyclopl egic autorefraction; none differed by 10 degrees or more. The least repeata ble measurement was noncycloplegic retinoscopy; 40% of the measurements dif fered by 10 degrees or more. CONCLUSION: For studies seeking to measure changes in astigmatism in normal eyes, cycloplegic autorefraction is the method of choice.