ETDRS Acuity Testing

ETDRS acuity testing has become the worldwide standard for visual acuity testing, replacing the Snellen and Sloan acuity tests.

ETDRS stands for Early Treatment Diabetic Retinopathy Study. The ETDRS test is designed to eliminate inaccuracies in the Snellen and Sloan tests.

The ETDRS acuity test was developed to aid in evaluating the changes in vision following panretinal photocoagulation in patients with diabetic retinopathy.

Previous studies had somewhat confusing results due to inadequacies of the acuity tests used in those early studies. For example, the Snellen acuity test has a different number of letters per row, e.g. 10 letters on the 20/20 line, but only 3 letters on the 20/70 line.

Therefore, if the results of a study showed that the patients “gained three letters of acuity,” the results could indicate the gain of a full acuity line, as in the 20/70 line, or only a portion of an acuity line, as in the 20/20 line.

Also, the individual lines on the Snellen acuity test are not equally spaced. For example, the change from the 20/25 line to the 20/20 line is a 20% change, while the change from the 20/30 line to the 20/25 line is a 16% change.

Again, referring back to the diabetic studies, if a study demonstrated a two line increase in acuity, it could mean a 33% improvement from 20/30 to 20/20, or a 40% improvement from 20/50 to 20/30. These inadequacies of the Snellen acuity test made it impossible to properly evaluate the acuity data and to compare data from study to study.

The ETDRS test incorporates specific design criteria to make it more accurate than the Snellen or Sloan acuity tests. These include:

• Same number of letters per row (5)
• Equal spacing of the rows on a log scale
• Equal spacing of the letters on a log scale
• Individual rows balanced for letter difficulty

To prevent memorization, different versions of the ETDRS test chart are available. The three standard versions of the ETDRS chart are R, 1 and 2.

For eye care professionals now are available the outstanding LCD Chart systems: unique solutions for both monocular and binocular tests.