The increasing number of autism cases seen in California since the 1990s is in large part real, not simply the result of changes in diagnostic criteria or in how autism cases are counted, new research suggests.

This study is the first to assess whether the autism trends in California might be explained by changes in age at diagnosis or by inclusion of milder cases, Dr. Irva Hertz-Picciotto and Dr. Lora Delwiche, from the University of California, Davis, note.

Using data from the California Department of Development Services, the researchers found that autism rates among children aged 5 years or younger rose steadily from 0.8 per 10,000 children born in California in 1990 to 11.2 per 10,000 children born in 2006.

The cumulative incidence per 10,000 births climbed from 6.2 in 1990 to 42.5 in 2001.

The proportion of cases that were diagnosed by 5 years of age rose only slightly from 54 percent to 61 percent for 1990 to 1996 births, according to a report in the January issue of Epidemiology.

A change in the age at diagnosis could explain 12 percent of the increase in autism rates, while inclusion of milder cases could explain 56 percent.

“With evidence of a leveling off, the possibility of a true increase in (autism) incidence deserves serious consideration,” the investigators emphasize.

“It’s time to start looking for the environmental culprits responsible for the remarkable increase in the rate of autism in California,” Hertz-Picciotto added in a statement.

SOURCE: Epidemiology, January 2009.