More than 10 percent of teens who said they were abstinent also tested positive for a sexually transmitted disease, says a study in Pediatrics released Monday — a figure public health advocates say justifies screening all teens for such diseases.

Study author Jessica McDermott Sales, a research assistant professor in the Department of Behavioral Sciences and Health Education at Emory University, said STD screenings should happen, regardless of what they say about their sexual histories.

“These infections can have some pretty major consequences for young people … . It’s worth the urine test,” she said.

The study was based on data from the National Longitudinal Study of Adolescent Health (“Add Health”), which has tracked thousands of teens and young adults for more than a decade.

In Wave 3 of Add Health, about 14,000 youth agreed to provide a urine specimen to check for three STDs — gonorrhea, chlamydia and trichomoniasis.

More than 11,000 of these youth said they had had sexual intercourse in the last 12 months, while the remaining 3,000 youth reported no sexual intercourse during that time.

The STD tests came back positive for 964 youth.

The vast majority of these STD results occurred in youth who said they had had sexual intercourse in the last year.

But 118 youth who denied having sexual intercourse also tested positive for an STD. In fact, 60 of these young people said they had never had sexual intercourse in their lives.

It is a “very important” finding that self-reported abstinent youth can still test positive for a STD, said Ms. Sales, who holds a doctorate in psychology cognition and development.

“If an individual goes to their doctor and says they are sexually active, that’s when the doctor may suggest that they get screened for STDs,” she said. But if someone says they are not sexually active, “it’s not the traditional practice to move forward with an STD test.”

The study’s co-authors, Ralph J. DiClemente, Fred Danner and Richard A. Crosby, took steps to be conservative in their findings, added Ms. Sales. But the reasonable explanations for why such a significant number of abstinent youth acquired an STD are that they forgot they had intercourse or they did not tell the truth about their sexual histories.

Today’s study is not the first to address STDs in abstinent youth.

In 2005, Peter Bearman and Hannah Bruckner, also using Add Health data, found that there was no significant difference — less than 3 percentage points — between STD rates among teens who pledged to be virgins until marriage and teens who did not make such a pledge.

The Bearman-Bruckner study, published in the Journal of Adolescent Health, suggested that virginity pledgers were becoming infected through other kinds of sexual activity, since some of them reported having oral sex or anal sex.

Heritage Foundation researchers Robert Rector and Kirk Johnson disputed the Bearman-Bruckner findings in a 2005 report, saying that if Add Health youth were compared on a wider range of measures — i.e., urine-sample results plus STD diagnoses and STD symptoms — the virginity pledgers were significantly less likely to have acquired an STD than nonpledgers.

Virginity pledgers, the Heritage researchers added, were also less likely to become pregnant as teens, have children out of wedlock, give birth as teens or young adults, have sex before age 18 or engage in nonmarital sex as young adults.

Regarding the new study, Ms. Sales said there was a “small possibility” that the 118 abstinent youth with an STD may have acquired their infections through noncoital activity. However, given the normal transmission routes of the three surveyed STDs, the majority “would have to have occurred through penile/vaginal sex,” she said.

The Pediatrics study offered caveats: Owing to the long interval between Wave 2 and Wave 3 of Add Health, a young person could have had sex and acquired an STD more than a year ago, and therefore answered truthfully about not having sex “in the last 12 months.” Also, there could have been false-positives in the Add Health STD data.

But when asked whether it’s just better to “trust, but verify,” Ms. Sales agreed that “that’s a safe approach to take” with STD screening.

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