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Lillian E. Agelis, Khaik K. Peh, Fred K. Chen; ETDRS letter score equivalents of Snellen fractions in 158 patients. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4787. doi: https://doi.org/.
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To report the Early Treatment Diabetic Retinopathy Study (ETDRS) letter chart scores for each Snellen fraction and compare the measured, with expected ETDRS scores for certain visual acuity (VA) levels on Snellen charts that have the same letter font sizes as those on the ETDRS charts.
This was a retrospective review of patient notes from the Royal Perth Hospital ophthalmology outpatient department. All patients had consecutive VA testing through a pin hole using a 6m Snellen chart and a 4m ETDRS chart in randomized order; by the same optometrist, in a standardized setting and using a standardized testing protocol for each chart. Thirty was added to the total number of letters read correctly on the ETDRS chart from 4 m to give the VA score. The score from the right eyes were chosen for analysis. Results from 47 patients were excluded because they were either unable to read all letters on the 6/24 line, or, were able to read all the letters on the 6/4 line. This eliminated a floor and ceiling effect of the ETDRS and the Snellen charts respectively. The Snellen fractions of remaining 158 patients were then divided into 6 groups i.e. Group 1 (6/24 to 6/24+3), Group 2 (6/18 to 6/18+4), Group 3 (6/12 to 6/12+5), Group 4 (6/9 to 6/9+6), Group 5 (6/6 to 6/6+7) and Group 6 (6/5 to 6/5+7). For each group the mean and standard Deviation (SD) of their ETDRS scores were calculated. Given that the font sizes for 6/24, 6/12 and 6/6 lines on the Snellen chart are the same as Rows 5, 8 and 11 of the ETDRS chart at 4m, a one sample t-test was used to compare the mean ETDRS scores from Groups 1,3 and 5 to the expected minimum ETDRS scores of 55, 70 and 85 respectively.
There were 104 males and 54 females with mean (SD) age of 61(18) years (range 18-93). The mean +SD ETDRS scores for each sub-group was 53±5 (n=15, Group 1); 63±8 (n=19, Group 2); 67±6 (n=31, Group 3); 75±8 (n=38, Group 4); 82±6 (n=28, Group 5) and 86±3 (n=27, Group 6). One-sample t-test, showed that the measured ETDRS score was significantly lower than the expected minimum ETDRS scores in two of the three groups (Group 1: p=0.14; Group 3: p=0.01 and Group 5: p=0.01).
Snellen fraction and ETDRS score are not readily interchangeable even when equivalent font size is taken into account. This has implications for VA analysis in retrospective studies that used VA measurements from both types of charts.
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