ATLANTA (AP) -- Autism cases are on the rise again, largely due to wider screening and better diagnosis, federal health officials said Thursday.

The rate of U.S. cases of autism and related disorders rose to about 1 in 88 children. The previous estimate was 1 in 110.

The new figure is from the latest in a series of studies that have been steadily increasing the government's autism estimate. This new number means autism is nearly twice as common as officials said it was only five years ago, and likely affects roughly 1 million U.S. children and teens.

Health officials attribute the increase largely to better recognition of cases, through wide screening and better diagnosis. But the search for the cause of autism is really only beginning, and officials acknowledge that other factors may be helping to drive up the numbers.

"We're not quite sure the reasons for the increase," said Coleen Boyle of the Centers for Disease Control and Prevention.

Autism is diagnosed by making judgments about a child's behavior; there are no blood or biologic tests. For decades, the diagnosis was given only to kids with severe language and social impairments and unusual, repetitious behaviors. The definition of autism has gradually expanded, and "autism" is now shorthand for a group of milder, related conditions, including Asperger's syndrome. Meanwhile, there's been an explosion in autism-related treatment and services for children.

As in the past, advocacy groups seized on the new numbers as further evidence that autism research and services should get greater emphasis. The new figures indicate "a public health emergency that demands immediate attention," said Geraldine Dawson, chief science officer for the advocacy group Autism Speaks.

The CDC study released Thursday is considered the most comprehensive U.S. investigation of autism prevalence to date. Researcher gathered data from areas in 14 states Alabama, Arizona, Arkansas, Colorado, Florida, Georgia, Maryland, Missouri, New Jersey, North Carolina, Pennsylvania, South Carolina, Utah and Wisconsin.

They looked specifically at 8-year-old children because most autism is diagnosed by that age. They checked health and school records to see which children met the criteria for autism, even if they hadn't been formally diagnosed. Then, the researchers calculated how common autism was in each place and overall.

An earlier report based on 2002 findings estimated that about 1 in 150 children that age had autism or a related disorder such as Asperger's. After seeing 2006 data, the figure was revised to about 1 in 110. The estimate released Thursday, based on 2008 data, is 1 in 88.

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Autism rates up; screening, better diagnosis cited