Is the Barrett True‑K formula the best method for calculating intraocular lens (IOL) power after laser-assisted in situ keratomileusis (LASIK)?

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Last updated: March 13, 2026 • View editorial policy

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Barrett True-K Formula for Post-LASIK IOL Power Calculation

The Barrett True-K formula is among the most accurate formulas for IOL power calculation after LASIK, performing equivalently to or better than other specialized post-refractive surgery formulas, though no single formula is definitively "best" for all cases. 1

Evidence Supporting Barrett True-K Performance

The Barrett True-K formula demonstrates strong performance in post-refractive surgery eyes:

  • In post-myopic LASIK/PRK eyes, Barrett True-K achieved the lowest mean absolute error (0.36 D) and the highest percentage of eyes within ±0.25 D (44.8%) compared to other formulas including Haigis-L. 1

  • For post-hyperopic eyes, Barrett True-K similarly showed the lowest mean absolute error (0.41 D) and highest percentage within ±0.25 D (42.6%), performing equivalently to the ASCRS-mean approach. 1

  • The American Academy of Ophthalmology recognizes Barrett Universal II as a specialized formula for post-keratorefractive surgery calculations, noting that Wang-Koch adjustments should NOT be used with it. 2

Comparative Performance with Other Methods

Barrett True-K vs. Other Formulas

  • A 2024 study found Barrett True-K with total keratometry (TK) had among the lowest median absolute errors (0.28 D), equivalent to the Shammas formula, in post-myopic laser refractive surgery eyes. 3

  • Barrett True-K TK demonstrated the lowest mean absolute error (0.59 D) and highest percentage within ±0.50 D (54.55%) overall in post-myopic LASIK/PRK eyes. 4

Barrett True-K vs. Intraoperative Aberrometry

However, newer evidence challenges Barrett True-K's supremacy:

  • A 2024 study of 1,067 eyes found intraoperative aberrometry (ORA System) provided significantly more accurate refractive predictability than Barrett True-K, with higher proportions achieving absolute prediction errors ≤0.25 D (P=0.0128) and ≤0.50 D (P<0.0001). 5

  • Mean and median absolute prediction errors were significantly lower with intraoperative aberrometry compared to Barrett True-K (P<0.0001). 5

Clinical Application Strategy

When to Use Barrett True-K

  • Barrett True-K embedded in modern biometers (IOLMaster 700) offers a streamlined, single-formula approach that reduces transcription errors from manual data entry required by multiple-formula calculators. 1

  • The formula performs consistently across different axial lengths in post-refractive surgery eyes. 5

Special Considerations for Corneal Characteristics

  • In eyes with low back-to-front corneal radius ratio (≤0.70) and flat posterior keratometry (≥-5.70 D), the Potvin-Hill formula may outperform Barrett True-K with lower mean absolute errors (0.46-0.67 D). 4

Alternative Approaches

  • Using total keratometry (TK) measurements with existing post-LASIK formulas has not demonstrated clear benefit and may require formula optimization. 6

  • The Wang-Koch-Maloney formula with total keratometry showed competitive results (57% within ±0.50 D) in some studies. 6

  • Intraoperative aberrometry should be considered when available, as it may provide superior accuracy to any preoperative formula in post-LASIK eyes. 5

Common Pitfalls to Avoid

  • Do not apply Wang-Koch axial length adjustments when using Barrett True-K or other specialized post-refractive surgery formulas, as this will compromise accuracy. 2

  • Avoid relying solely on anterior keratometry measurements when posterior corneal measurements are available, as incorporating posterior corneal data can improve outcomes. 4

  • Do not assume one formula will be optimal for all post-refractive surgery cases—consider patient-specific corneal characteristics when selecting between formulas. 4

References

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.