Snellen Visual Acuity May Affect ETDRS Outcomes in Clinical Trials

Snellen visual acuity measurements used in clinical practice and ETDRS letters used in clinical trials may show some discrepancies in clinical trials.

Baseline Snellen visual acuity, which is often converted to an Early Treatment Diabetic Retinopathy Study (ETDRS) equivalent when comparing clinically-obtained data with clinical trials, shows a similar change during the course of clinical trials compared with true ETDRS visual acuity, according to research published in Ophthalmology Science. However, patients with worse baseline Snellen vision experience greater changes in Snellen vision compared with ETDRS. Conversely, those with better baseline Snellen vision experience greater changes with the ETDRS method.

Researchers included patients enrolled in prospective clinical trials from 3 retina practices (N=273; mean age, 68.7 years; 48.1% women) in the retrospective investigation. During a data collection period that took place between January 2006 and March 2021, the team compared Snellen visual acuity and ETDRS visual acuity and evaluated correlations and discrepancies between the methods from trial entry to exit.

The mean Snellen VA was 20/69 (ETDRS equivalent, 58.1 letters) before trial entry and 20/59 (ETDRS equivalent, 61.6 letters) after trial exit. The true ETDRS visual acuity was 65.5 letters (Snellen 20/49) at trial entry and 70.5 letters (Snellen 20/39) at trial exit, the report shows. 

Overall, there was no significant difference in mean visual acuity change from trial entry to exit between the ETDRS and Snellen methods (5 vs 3.6 letter gain; P =.061). However, eyes with baseline Snellen visual acuity of 20/50 or worse gained significantly more letters using the ETDRS equivalent compared with the true ETDRS (9.3 vs 5.2 letters, P=.012) and eyes with baseline Snellen visual acuity better than 20/50 gained significantly more letters with ETDRS compared with the Snellen, ETDRS equivalent (4.9 vs -1.5 letters; P <.001).

“These findings may have implications for interpreting real-world [visual acuity] outcomes compared to clinical trials,” the study authors explain. “In particular, the proportion of patients included in real-world studies with better versus worse Snellen vision at baseline should be analyzed as this metric may influence the reported magnitude of [visual acuity] change,”

Study limitations include a retrospective nature, non standardized Snellen assessments, and the inclusion of patients with different retinal diseases, which may cause discrepancies in ETDRS and Snellen visual acuity.

Disclosure:  Some study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of authors’ disclosures. 


Salabati M, Huang C, Kamalipour A, et al. Magnitude of visual acuity change with ETDRS vs Snellen testing in clinical trials: implications for real world outcomes. Ophthalmol Sci. Published online July 18, 2023. doi:10.1016/j.xops.2023.100372