Chicago ? It’s supposed to be this simple: A teenager goes in for an annual physical, and at some point the doctor says: “HIV testing is a routine part of the exam. Would you rather not be tested?”

But almost four years after federal officials urged that routine HIV testing begin at age 13, unless the patient declines, experts say many health care providers who treat teenagers have not adopted the recommendations.

“Most clinicians are not aware of the guidelines and they’re not being implemented,” said Dr. Jaime Martinez, associate professor of pediatrics at the University of Illinois at Chicago.

Meanwhile, Americans ages 13 to 29 represent more new HIV infections than any other age group, making up 34 percent of new infections documented by the U.S. Centers for Disease Control and Prevention in 2006.

Yet about 48 percent of adolescents infected with HIV are unaware of it, the CDC estimates, compared with 21 percent of all HIV-infected Americans.

Adolescents with HIV often become seriously ill before they are diagnosed, Martinez said, and those unaware of their infections are less likely to practice safe sex and more likely to transmit the infection.

“Youth are contracting it quicker and finding out older. By that time it’s a really bad health issue,” said Anthony Galloway of Test Positive Aware Network, a community health group.

Such concerns led the CDC in 2006 to urge all health care providers to offer patients ages 13 to 64 an HIV test on a routine basis. The agency previously had supported pretest counseling and explicit consent arrangements, often in writing, while focusing HIV testing efforts on high-risk populations.

The American Academy of Pediatrics’ Committee on Pediatric AIDS is developing a new policy statement, now under review, to inform and guide pediatricians about adolescent HIV testing.

“The consensus among experts is that HIV testing be widely available for youth in agreement with the CDC recommendations,” said Dr. Patricia Flynn, chairwoman of the committee and a member of the Department of Infectious Diseases at St. Jude Children’s Research Hospital in Memphis.

When the CDC recommended that testing start at 13, “it shocked the medical community,” Flynn said. Many doctors thought parents would object to the CDC’s guidelines and patients would not want to be tested, she said.

But Flynn and colleagues reported in the journal Pediatrics last year that most patients and parents in a large Memphis pediatric emergency department consented to the test when it was offered.

“My experience is that the overwhelming majority — 90-plus percent — say ‘Fine, go ahead and do it,'” said Dr. Daniel Johnson, an infectious disease specialist at the University of Chicago.