Purpose/Aim: To evaluate the best formula to predict intraocular pressure (IOP) reduction after cataract surgery. Materials and methods: In this prospective longitudinal study, we included consecutive patients with or without glaucoma, either with open or with narrow angles but without peripheral anterior synechiae, who underwent phacoemulsification. Clinical factors and anterior segment parameters measured with optical biometry and optical coherence tomography were evaluated as preoperative predictors for IOP reduction at 6 months postoperatively. To find the best combinations of predictors, model selection was conducted using least absolute selection and shrinkage operator regression with cross-validation. Results: A total of 156 eyes from 109 patients were enrolled. The mean age of the patients was 74.89 (±8.54) years and the average preoperative IOP was 15.6 (±3.68) mmHg with 0.7 (range 0-4) glaucoma medications. The mean IOP reduction after phaco was 3.1 (±2.49) mmHg at postoperative month 6. After multivariate analysis, preoperative IOP (β = 0.49 [0.4-0.59] P < .0001), gonioscopy score (β = -0.14 [-0.23 to -0.06] P = .0001), anterior chamber depth (β = -0.85 [-1.64 to -0.07] P = 0.033), lens thickness (LT) (β = 0.87 [0.12-1.62] P = .024), and angle open distance 750 (β = -2.2 [-3.96 to -0.44] P = 0.014) were associated with IOP reduction. The best formula to correlate with IOP reduction was -4.76 + (0.46 × preoperative IOP) - (0.42 × LT) - (0.1 × gonioscopy score) - (0.66 × glaucoma status). Conclusion: Different formulas were developed and were able to improve the predictive value of the preoperative IOP. When used in combination, these formulas may help the clinician to know how much IOP reduction to expect after cataract surgery at 6 months postoperatively.

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