

LETTER TO THE EDITOR 

Year : 2020  Volume
: 68
 Issue : 7  Page : 1507 

Comments on: Evaluation of Barrett universal II formula for intraocular lens power calculation in Asian Indian population
Ankur K Shrivastava, Swatishree Nayak
Department of Ophthalmology, AIIMS, Raipur, Chhattisgarh, India
Date of Web Publication  25Jun2020 
Correspondence Address: Dr. Ankur K Shrivastava 603/Type 5A, AIIMS Residential Complex, Kabir Nagar, Raipur, Chhattisgarh India
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/ijo.IJO_190_20
How to cite this article: Shrivastava AK, Nayak S. Comments on: Evaluation of Barrett universal II formula for intraocular lens power calculation in Asian Indian population. Indian J Ophthalmol 2020;68:1507 
How to cite this URL: Shrivastava AK, Nayak S. Comments on: Evaluation of Barrett universal II formula for intraocular lens power calculation in Asian Indian population. Indian J Ophthalmol [serial online] 2020 [cited 2020 Sep 30];68:1507. Available from: http://www.ijo.in/text.asp?2020/68/7/1507/287542 
Dear Editor:
It was interesting to read the article “Evaluation of Barrett universal II formula for intraocular lens power calculation in the Asian Indian population” by Vanathi et al.^{[1]} The authors have compared four formulas and concluded that Barret Universal II (BU) was most accurate in the prediction of postoperative refraction over a wide range of axial lengths.
However, we wish to point out certain concerns that we thought were important.
 The authors have compared four formulas: modified SRK II, SRK/T, Olsen, and BU II. As per the Hoffer K J protocols, outdated regression formulas such as SRK I and SRK II should not be used while doing comparative study on intraocular lens (IOL) power calculation as they are errorprone ^{[2]}
 Among recent formulas, authors have only compared the accuracy of BU II and Olsen. Many of the newer fourth and fifthgeneration formulas like Haigis, Holladay 2, and HillRBF are being widely used recently. The inclusion of these in the comparative analysis might have generated different results
 IOL power was calculated using Mod SRK II and BU II; in cases where the difference in calculated IOL power was more than 0.5 D between two formulas, IOL was implanted according to the SRK/T value.^{[1]} As the aim of the study was to evaluate the accuracy of the BUII formula in predicting the IOL power for cataract surgery, the authors should have used BU II for IOL implantation rather than SRK/T
 The authors have analyzed mean absolute prediction error as a marker of accuracy as suggested by Aristodemou et al.^{[3]} Because absolute errors do not have a normal Gaussian distribution and mean absolute error is influenced by outliers, both Hoffer et al. and Wang et al. have proposed that median absolute error should replace the mean absolute error ^{[2],[4]}
 Lens constant optimization is mandatory for any clinical study comparing IOL power calculation formulas as otherwise, the results would be skewed toward a single formula.^{[2],[4]} This optimization has not been done in the present study
 [Figure 1] shows the percentage of eyes with prediction error only within ±0.5 D and ±1 D of the given target postoperative refraction. However, Hoffer et al. emphasized that the percentage of eyes within ±0.50 D, ±1.00 D and ±2.00 D of refractive prediction errors should be reported. With recent advances and better refractive outcomes, the present trend is also to report the percentages of eyes within ±0.25 D as suggested by Wang et al.^{[4]}
 Figure 1: Percentage of eyes with refractive prediction errors within ± 0.25 D, ± 0.50 D, ± 1.00 D and ± 2.00 D
Click here to view 
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References   
1.  Vanathi M, Kuthirummal N, Mukhija R, Gupta N, Meel R, Saxena R, et al. Evaluation of Barrett universal II formula for intraocular lens power calculation in Asian Indian population. Indian J Ophthalmol 2020;68:5964. [ PUBMED] [Full text] 
2.  Hoffer KJ, Aramberri J, Haigis W, Olsen T, Savini G, Shammas HJ, et al. Protocols for studies of intraocular lens formula accuracy. Am J Ophthalmol 2015;160:4035. 
3.  Aristodemou P, Knox Cartwright NE, Sparrow JM, Johnston RL. Statistical analysis for studies of intraocular lens formula accuracy. Am J Ophthalmol 2015;160:10856. 
4.  Wang L, Koch D, Hill W, Abulafia A. Pursuing perfection in intraocular lens calculations: III. Criteria for analysing outcomes. J Cataract Refract Surg 2017;43:9991002. 
[Figure 1]
