BOSTON -- A child may be able to read an eye chart and pass with flying colors, but that routine screening may not be showing parents everything.

Margaret Thornton said she is used to trips to the eye doctor.

"It goes really quick and easy and there's not much that happens that's not regular," she said.

Diagnosed with an eye condition at a young age, her mom also learned early on that you can't always trust the eye chart.

"Sometimes you don't know if you are getting accurate information," Margaret's mother, Jennifer Thornton, said.

"You take your eye test at school, and if you pass that means you can see. That's not always the case. There are some kids who pass and fall through the cracks," Children's Hospital Boston's Dr. David Hunter said.

Hunter said new research is showing that while school eye tests almost always catch nearsightedness, almost 50 percent of children who are farsighted or have astigmatisms are going undetected.

"Even if they pass at school or pediatricians office, it may be worth getting eye exam to make sure they don't have more rare conditions that may be interfering with learning," Hunter said.

Hunter said to look for warning signs, such as trouble in school, not wanting to read or reading with a book too close to the face.

Massachusetts is one of a handful of states that mandates that pediatricians test a child's eyes before they enter kindergarten, but even that is not always enough.

In some cases, lazy eye, or amblyopia, will have no visible signs in pre-school aged children, making diagnosis almost impossible.

"If only we picked it up a few years earlier when they were 3 or 4 then it's too late to catch up," Hunter said.