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Vision 101: Understanding the Threshold and Critical Line Screening Methods

In the last blog of our Vision 101 series, we told you what features make a proper vision chart for children over the age of 6. This time, we will review two methods for screening distance acuity in children over the age of 6 (or the age of 5, in some states).

There are several methods that can be used depending on the state, local or district guidelines. Below we highlight the two main methods, Threshold Screening and Critical Line Screening.

Threshold Screening Method

Threshold Screening is the traditional method of testing visual acuity. Here’s how it works: ask the child to start reading at the top of the eye chart and continue reading down each line until he or she recites the smallest line of optotypes discernable. Each eye is tested separately (using frosted occluders), thus allowing you to identify the best level of visual acuity in each eye. Refer the child to an eye doctor if there is a two-line difference in either eye (even in the passing range) or acuity of 20/40 (or worse) in either eye.

Critical Line Screening Method

Because younger children, even those with normal vision, are frequently unable to recognize small optotypes and identify them, Critical Line Screening is an effective alternative. Here’s how it works: the “critical line” is the age-dependent line a child is expected to see normally. It’s not necessary to measure acuity below the age-specific critical line to pass the test. Screen each eye individually (monocularly) and note that the critical line to pass becomes smaller as age increases. Children ages 6 years (or the age of 5, in some states) and older should pass the majority of optotypes on the 20/32 line. Refer the child to an eye doctor for acuity of 20/40 (or worse) in either eye.

To shop MacGill’s selection of Threshold Screening charts, click here.
To shop MacGill’s selection of Critical Line Screening charts and flipbooks, click here.

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