By Sean Henahan, Access Excellence

BALTIMORE, Md. (9/24/96) A new DNA-based test for chlamydia infection is almost too sensitive for its own good, reported researchers from Johns Hopkins at the 36th Interscience Conference on Antimicrobial Agents and Chemotherapy.

Chlamydia is a common infection of the urinary tract. Infecting an estimated four million young adults in the United States, chlamydia is the most common sexually transmitted disease.The new urine test uses a technique known as DNA amlification and is so sensitive it can detect the genetic footprints of the microorganisms that cause inefction 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 Dr. Charlotte Gaydos, assistant professor of medicine at Johns Hopkins. It takes that long for the chlamydia DNA to clear completely from cells that had been infected, and which accumulate in the urine.

New research 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, according to Gaydos.

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.The Hopkins team used two DNA amplification tests, ligase chain reaction and polymerase chain reaction

"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. For two weeks after treatment, doctors should avoid using this technique to test urine samples to ensure they get accurate results."