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One-Compartment Open Model: Wagner-Nelson and Loo Riegelman Method for ka Estimation01:24

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This lesson introduces two critical methods in pharmacokinetics, the Wagner-Nelson and Loo-Riegelman methods, used for estimating the absorption rate constant (ka) for drugs administered via non-intravenous routes. The Wagner-Nelson method relates ka to the plasma concentration derived from the slope of a semilog percent unabsorbed time plot. However, it is limited to drugs with one-compartment kinetics and can be impacted by factors like gastrointestinal motility or enzymatic degradation.
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Updated: Nov 26, 2025

Author Spotlight: Advancements in Refractive Surgical Correction for Presbyopia and Exploring Postoperative Visual Acuity
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IOL Power Calculation After Radial Keratotomy Using the Haigis and Barrett True-K Formulas.

Renata Leite de Pinho Tavares, Gabriel de Almeida Ferreira, Vinícius Coral Ghanem

    Journal of Refractive Surgery (Thorofare, N.J. : 1995)
    |December 9, 2020
    PubMed
    Summary

    The Barrett True-K formula offers improved refractive prediction accuracy for intraocular lens (IOL) power calculations after radial keratotomy compared to the Haigis formula. However, it may yield hyperopic results in very flat corneas.

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    Area of Science:

    • Ophthalmology
    • Refractive Surgery
    • Biomedical Engineering

    Background:

    • Radial keratotomy (RK) alters corneal curvature, complicating intraocular lens (IOL) power calculations.
    • Accurate IOL power calculation is crucial for achieving desired refractive outcomes post-cataract surgery.
    • Traditional formulas may struggle with post-RK eyes, necessitating specialized approaches.

    Purpose of the Study:

    • To compare the accuracy of the Haigis and Barrett True-K formulas for IOL power calculation in patients with prior RK.
    • To evaluate refractive prediction errors using both formulas in this specific patient cohort.

    Main Methods:

    • A retrospective case series analyzed 108 eyes of patients with previous RK.
    • Preoperative data from IOLMaster 500/700, implanted IOL power, and postoperative refraction were used.
    • Refractive prediction error was calculated to assess formula accuracy.

    Main Results:

    • The Barrett True-K formula demonstrated significantly better arithmetic predictability (-0.03 ± 0.96 D) than the Haigis formula (-0.29 ± 1.00 D).
    • No significant differences were found in mean absolute refractive prediction error or the percentage of eyes within ±0.50/±1.00 D between the formulas.
    • Subgroup analysis indicated the Barrett True-K formula tended towards hyperopic results in very flat corneas (K1 < 38.00 D).

    Conclusions:

    • The Barrett True-K formula provides superior arithmetic predictability for IOL power calculation in post-RK eyes.
    • Ophthalmologists should be aware of the Barrett True-K formula's potential for hyperopic shifts in patients with very flat corneas.
    • Further refinement of IOL calculation methods for post-RK eyes is warranted.