Chlamydia rates in the United States. (Source: CDC)

Doctors may be failing to detect large numbers of gonorrhea and chlamydia infections in women if they don't test patients' throats and rectums for the sexually transmitted diseases, researchers at Johns Hopkins University report.

Current guidelines from the Centers for Disease Control and Prevention call for rectal and pharyngeal screening for the two infections only in HIV-infected men (and men at risk for HIV) who have sex with other men. But when the researchers looked at the health records of 4,402 women at two Baltimore clinics for sexually-transmitted diseases, they found that large numbers of cases would have been missed if "extragenital testing" had not been conducted.

All the women (as well as nearly 6,000 men in the study) had reported extragenital exposure during sex. The researchers determined that 30.3 percent of gonorrhea infections and 13.8 percent of chlamydia cases would have been missed if the tests hadn't been performed.

"Most studies suggest that 20 percent to 40 percent of [gonorrhea] infections and 10 percent to 25 percent of [chlamydia] infections would have been missed if extragenital testing were not done," the researchers reported. The study was published in the May issue of the journal Sexually Transmitted Diseases.

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The single best predictor of these kinds of infections among the women and girls tested was youth. "Age [of] less than 18 years was significantly associated with isolated [gonorrhea] of the throat and [chlamydia] of the rectum," the researchers, led by Khalil G. Ghanem and Joshua D. Trebach, wrote.

What isn't clear, however, is whether routine testing for extragenital gonorrhea and chlamydia infections is cost-effective. Because the infections are so much more common in men who have sex with men, finding a single additional case would require testing six to 10 times as many heterosexual women as men who have sex with men, the research shows. The researchers called for more study of cost-effectiveness.

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The study also found that heterosexual men had rates of 2.6 percent for extragenital gonorrhea and 1.6 percent for extragenital chlamydia. But for cost reasons, the two clinics did not test them for genital chlamydia, so comparisons could not be made.