Modifications made to the refractive result when prescribing spectacles.

Purpose: The purpose of this study was to determine how optometric practitioners modify the subjective refractive result when prescribing spectacles.

Methods: Refractive data were gathered for patient visits at the School of Optometry, University of Waterloo, between January 2007 and January 2008. The entering prescription, subjective refraction, and exiting prescription were analyzed from 5001 records for patients aged ≥ 7 years.

Results: The refraction was modified to create the prescription in at least one eye in 45% of cases; specifically, 27% of cases for the sphere power, 18% for the cylinder power, 25% for the cylinder axis, and 21% for the add. Significant differences, defined as ≥ 0.50 D in sphere, cylinder, or add power or a change in axis of 15° for cylinders < 1 D, 10° for cylinders between 1 and ≤ 2, and 5° for cylinders >2, were made in at least one eye in 17% of cases; specifically 9% of cases for the sphere power, 6% for the cylinder power, 6% for the cylinder axis, and 5% for the add. Spheres were more likely to be modified in the minus direction (weaker plus and stronger minus power) (18 vs. 11%), cylinder powers reduced (14 vs. 5%), and adds increased in power (12 vs. 8%). Modifications to create the exiting prescription were made to be closer or the same as the entering prescription 97% of the time. However, modifications were such that the entering prescription was duplicated exactly only 0.7% of the time.

Conclusions: Optometric practitioners routinely modify the subjective refraction to create the prescription. Small modifications are common, whereas larger modifications are used more sparingly. Because there is a significant amount of clinical judgment involved in determining the refractive prescription, reliance on automated or subjective refraction alone would not be prudent.