Improving the prediction accuracy of the SRK/T formula: the T2 formula

Purpose: To investigate the causes of nonphysiologic behavior of the SRK/T formula, assess their clinical significance, and develop and evaluate solutions.

Setting: Two NHS ophthalmology departments, United Kingdom, and a private practice, United States.

Design: Evaluation of technology.

Methods: The individual steps of the SRK/T formula were examined for nonphysiologic behavior, and the clinical significance of behaviors was assessed with reference to a database of biometry and refractive outcomes in 11 189 eyes. The full data set was divided into 2 subsets, the first to develop solutions to nonphysiologic behavior of the SRK/T formula and the second to evaluate their performance.

Results: The SRK/T formula showed nonphysiologic behavior in the calculation of corrected axial length and corneal height. Although the former is of little clinical significance, the latter showed a systematic error that contributes to inaccurate intraocular lens (IOL) power prediction. The T2 formula was developed using a regression formula for corneal height derived from the development subset. Comparison of the performance of the T2 and SRK/T formulas using the evaluation subset showed significant improvement in the mean absolute error with the T2 formula (0.3064 diopter [D] versus 0.3229 D; P<.0001). On average, the prediction error with the T2 formula was 9.7% less than with the SRK/T formula, with significantly higher proportions of eyes within ±0.50 D of target (P<.0001).

Conclusions: The SRK/T formula has nonphysiologic behavior that contributes to IOL power prediction errors. A modification to the formula algorithm, the T2 formula, can be directly substituted for SRK/T, resulting in significantly improved prediction accuracy.