DNA TEST CAN BE SO SENSITIVE IT FOOLS DOCTORS

August 21, 1996
Media Contact: Marc Kusintz
Phone: (410) 955-8665
E-mail: mkusinitz@welchlink.welch.jhu.edu

Scientists at Johns Hopkins say that a new urine test for chlamydia infection is so sensitive it can detect the genetic footprints of the germs that cause it up to two weeks after successful treatment with antibiotics.

"This means that doctors should wait no less than two weeks after patients finish antibiotic treatment before using this test to verify treatment success," according to Charlotte Gaydos, Dr. P.H., assistant professor of medicine. It takes that long, she says, for the chlamydia DNA to clear completely from cells that had been infected, and which accumulate in the urine.

Infecting an estimated four million young adults in the United States, chlamydia is the most common sexually transmitted disease. Gaydos says her study suggests that failure to wait long enough after therapy to retest urine by the DNA test may cause a positive test because of the DNA from dead cells, rather than from live organisms.

Simpler and more convenient than taking small scrapings of cells from a woman's cervix or a swab from a man's urethra, the tests use a technology called DNA amplification. Like a super-copying machine for genes, it produces millions of copies of genetic material found in the Chlamydia trachomatis organism, making it more easily detectable in the laboratory.

"For two weeks after treatment, doctors should avoid using this technique to test urine samples to ensure they get accurate results," says Gaydos, who is scheduled to present the findings of this study at the 36th annual meeting of the Interscience Conference on Antimicrobial Agents and Chemotherapy in New Orleans at 9 a.m., Sept. 17.

The Hopkins team used two DNA amplification tests, ligase chain reaction (LCR; Abbott Laboratories) and polymerase chain reaction (PCR; Roche Diagnostic Systems) to detect the organism in urine and cervical swab samples from 408 young women.

The PCR test detected chlamydia in cervical samples from 53 (13 percent) of the women. The same test for urine samples was positive for 63 (15.4 percent) of the women. The urine-LCR test was positive for 60 (14.7 percent) of urine samples.

"The urine DNA amplification tests were consistently more sensitive than the cervical tests," says Gaydos. "They continued to detect chlamydia DNA up to nine days--and sometimes up to two weeks--after treatment."

Other authors of the study include Kimberly A. Crotchfelt and Susan Kralian and Laura Welsh (Johns Hopkins); Pat Hauptman (Baltimore City Health Department), the Baltimore City Health Department Study Team; and Thomas Quinn (Johns Hopkins and the National Institutes of Health).


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