SRK/T Formula: A Review

Since 1975, IOL power has been calculated using accurate measurement of an eye’s corneal power and axial length (AL). Prior to that, the power of the IOL was calculated using clinical history alone—in other words, the preoperative refractive error prior to cataract development.

The earliest IOL power calculation formulas, in the late 1970s and early 1980s, were either theoretical or regression formulas. Regression formulas topped surgeons’ preferences, and one of the most successful was the SRK formula devised by Donald R. Sanders, PhD, MD; John A. Retzlaff, MD; and Manus C. Kraff, MD.^1,2

The SRK formula uses the following equation to calculate IOL power: P = A – BL - CK, where P is the implant power for emmetropia; L is the axial length (mm); K is the average keratometry (D); and A, B, and C are constants. The values of B and C are 2.5 and 0.9, respectively, and the value of A varies with the IOL design and the manufacturer. With this information, the formula can be written as follows: P = A – 2.5L - 0.9K.

Over the years, surgeons discovered that the SRK formula is best used in eyes with average AL, between 22.00 and 24.50 mm; a subsequent formula, the SRK II, was developed for use in long and short eyes.3 In this formula, a correction factor was added to increase the lens power in short eyes and decrease it in long eyes: P = A1 – 0.9K -2.5L. For eyes with AL of less than 20.00 mm, a numerical value of 3.00 is added to the A constant; a numerical value of 2.00 is added if the AL measures between 20.00 and 20.99, a numerical value of 1.00 if the measurement is between 21.00 and 21.99, and -0.50 if the AL is greater than 24.50 mm.

Even more customized formulas are required today to calculate anterior chamber depth (ACD) based on AL and corneal curvature. The SRK/T (T for theoretical) is one such formula, representing a combination of linear regression method with a theoretical eye model.⁴ Based on the nonlinear terms of the theoretical formulas, the SRK/T also incorporates empirical regression methodology for optimization, resulting in greater accuracy. The SRK/T and other third-generation formulas work best for near-schematic eye measurements; specifically, the SRK/T is best for eyes longer than 26.00 mm. With this generation, which used an iterative process on five data sets consisting of 1,677 procedures, the SRK/T formula optimizes the prediction of postoperative ACD, retinal thickness AL correction, and corneal refractive index. It can be calculated using the same A constants used with the original SRK formula or with ACD estimates.⁴ This calculation, however, does not account for effective lens position.

1. Sanders D, Retzlaff J, Kraff M, et al. Comparison of the accuracy of the Binkhorst, Colenbrander, and SRK implant power prediction formulas. J Am Intraocul Implant Soc. 1981;7(4):337-340.
2. Sanders DR, Retzlaff J, Kraff MC. Comparison of empirically derived and theoretical aphakic refraction formulas. Arch Ophthalmol. 1983;101(6):965-967.
3. Dang MS, Raj PP. SRK II formula in the calculation of intraocular lens power. Br J Ophthalmol. 1989.
4. Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg. 1990.