In optical coherence tomography (OCT) biometry for IOL calculation, the Barrett Universal II formula consistently outperforms third-generation formulas for axial lengths outside the normal range. The key variable that Barrett Universal II additionally incorporates compared to SRK/T is:
- A Lens thickness (LT) and white-to-white (WTW) corneal diameter in addition to ACD to predict lens factor (effective lens position) ✓
- B Anterior chamber depth (ACD) alone
- C Posterior corneal curvature measured by Scheimpflug imaging
- D Vitreous chamber depth measured by A-scan ultrasonography
Explanation
The Barrett Universal II formula uses lens thickness (LT), anterior chamber depth (ACD), white-to-white diameter (WTW), and axial length plus keratometry to predict the effective lens position (ELP) more accurately. Third-generation formulas (SRK/T, Hoffer Q, Holladay 1) primarily use axial length and keratometry to predict ELP, which introduces error in extreme eyes (short < 22 mm or long > 26 mm axial length). By incorporating preoperative lens thickness and ACD, Barrett Universal II better estimates where the IOL will actually sit postoperatively (ELP), reducing prediction error. The formula also uses the concept of a 'Lens Factor' (unique to Barrett formulas) analogous to the A-constant but with improved anatomical correlation.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.