Many teenagers and young adults may make overly optimistic estimates of how often they use a condom during sex, a new study finds.

Researchers found that among 715 young African-American women, many of those who said they’d consistently used a condom over the past two weeks had objective evidence that this was not the case. One-third had evidence of sperm DNA in samples of their vaginal fluid.

The findings have implications for young people’s sexual health, as well as studies on the matter, researchers say.

Studies that use both self-reports and objective measures of condom use may offer a clearer picture of young people’s sexual behavior and risks of sexually transmitted diseases (STDs), according to the researchers, led by Eve Rose of Emory University in Atlanta.

The study included African-American women between the ages of 15 and 21 who were enrolled in an HIV-prevention program. They were asked how many times they’d had sex in the past two weeks and how many times they had used a condom. They also provided vaginal-fluid samples to be tested for Y-chromosome DNA - evidence of sperm.

Of the women who’d had sex in the past two weeks, 186 said they’d used a condom every time. However, 34 percent of these women had Y- chromosome DNA in their fluid samples.

The reasons for the discrepancy - whether the women were mistaken, had misused the condoms, or had given the “socially desirable” response when asked about condom use - are unknown.

In the Archives of Pediatrics & Adolescent Medicine, Rose and her colleagues make the point that “regardless of whether the problem is condom user error or misreporting, the unfortunate result, in terms of risk for STDs and HIV, is the same.”

SOURCE: Archives of Pediatrics & Adolescent Medicine, January 2009.