Although refractive errors are present in nearly 30% of children and other vision problems, such as amblyopia and strabismus, affect an additional 4% of kids, vision screening requirements for school-aged students vary dramatically across the United States, new research suggests.

The investigative team found that 40 states and the District of Columbia mandate vision screening for school-aged children at least once, either in a school or community setting. Of this group, 13 states also include “language for recommended or optional screenings for additional grades.” Four states—Montana, Missouri, South Carolina and New Hampshire—recommend a vision screening but do not mandate one.

On the other hand, Alabama, Idaho, North Dakota, South Dakota, Wisconsin and Wyoming had no vision screening requirement or recommendation based on the review.

Not surprising, the study found screening is more commonly required in elementary school. All states with vision screening laws require screenings for kindergarten students either during or upon entry to that grade. Thirty states require a screening for at least one grade in middle school, and 19 states have vision screening on the books for at least one grade in high school.

Whether a state requires school-age vision screening may be based on its geographic region. Of the six states that require vision screening annually or every two years, four are clustered in the northeast United States. Similarly, five states with no vision screening requirements border one another in the north-central and northwest regions of the country.

Regarding testing, distance acuity is the most commonly required measure in 41 states, followed by color vision in 11 states. Near vision is performed in 10 states.

As only six states necessitate a vision screening annually or every two years, there is an urgent need to align state vision screening practices, especially in impoverished communities, the researchers noted.

“Adopting a universal strategy for vision screening in school-aged children at least every other year, which at present only a minority of states require, along with post-screening evaluation in schools, would be reasonable given the high rates of uncorrected vision problems in school-aged children,” the investigators wrote in their paper.