Conducting a Snellen Test

The visual acuity test should be performed in a well-lit room that is free of distractions. The test is usually performed at a distance of 20 feet; this can be conveniently marked off in the medical office with paint or a piece of tape so that it does not have to be remeasured every time the test is given.

At the side of each row of letters on the chart are two numbers, separated by a line. The number above the line represents the distance (in feet) at which the test is con­ducted. It is usually 20 because most eye tests are con­ducted at this distance. The number below the line repres­ents the distance from which a person with normal visual acuity can read the row of letters. The line marked 20/20 indicates normal distance visual acuity, or 20/20 vision. This means a person could read what he or she was sup­posed to read at a distance of 20 feet.

A visual acuity reading of 20/30 means this was the smallest line that the individual could read at a distance of 20 feet. People with normal acuity would be able to read this line at a distance of 30 feet.

A visual acuity reading of 20/15 means this was the smallest line that the individual could read at a distance of 20 feet. It indicates above-average acuity for distance vi­sion. People with normal acuity would be able to read this line at 15 feet.

The acuity of each eye should be measured separately, traditionally beginning with the right eye. Most physicians prefer that the patient wear his or her contact lenses or glasses, except reading glasses, during the test; the medical assistant should record in the patient's chart that corrective lenses were worn by the patient during the test. An eye oc­cluder should be held over the eye not being tested. The patient's hands should not be used to cover the eye be­cause this may encourage peeking through the fingers, especially in the case of children. The patient should be in­structed to leave open the eye not being tested because closing it causes squinting of the eye that is being tested. The procedure for measuring distance visual acuity is out­lined in Procedure 6-1