Formula Discordance and Refractive Target Error: Temporal Validation of Legacy–Modern Intraocular Lens Formula Disagreement as a Quality-Control Marker

PURPOSE: To determine whether disagreement between third-generation and modern intraocular lens (IOL) formulas identifies eyes at increased risk of refractive target error. SETTING: Yenikent State Hospital, Sakarya, Türkiye. DESIGN: Retrospective cohort with chronological derivation and temporal validation. METHODS: Six hundred eyes with foldable one-piece IOL implantation were analyzed. Third-generation formulas were SRK/T, Hoffer Q, and Holladay 1; modern formulas were Barrett Universal II, Kane, EVO 2.0, PEARL-DGS, and Cooke K6. Formula-specific offsets were estimated in 400 earlier eyes and frozen for 200 later validation eyes. The Legacy-Modern Gap (LMG) was the absolute difference between mean predicted postoperative spherical equivalent of the 2 formula groups. The primary endpoint was an absolute target miss of at least 0.50 D. RESULTS: Target miss occurred in 29.0% of derivation eyes and 26.0% of validation eyes. Validation mean absolute error was 0.284-0.317 D for modern formulas and 0.358-0.424 D for third-generation formulas (P<0.001). Each 0.10 D increase in LMG was associated with target error (adjusted odds ratio, 1.21; 95% CI, 1.05-1.38; P=0.007). Validation AUC was 0.650 versus 0.603 for biometrics alone (ΔAUC, 0.047; 95% CI, -0.015 to 0.111). Brier score improved by 0.0116 (95% CI, 0.0034-0.0200); calibration intercept was 0.01 and slope 1.25. CONCLUSIONS: Formula disagreement added information beyond biometrics but had modest discrimination. LMG is a supplementary quality-control signal rather than an independent rule for IOL selection.