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Ease of New DNA Assays May Lead to Overuse.

ATLANTA -- DNA amplification methods of testing patients for Chlamydia trachomatis are far easier than traditional tissue culture methods, but their simplicity may be luring physicians to use them inappropriately, Dr. Michael Rein warned at the annual meeting of the American College of Physicians--American Society of Internal Medicine. Interest in DNA amplification methods that use urine or self-collected vaginal specimens to test for C. trachomatis is picking up steam, because they're cost effective and allow for the elimination of invasive genital and pelvic examinations, said Dr. Rein, an infectious disease expert from the University of Virginia, Charlottesville.

At the same time, both ligase chain reaction (LCR) and polymerase chain reaction (PCR) methods are at least as sensitive as traditional tissue culturing methods, which have a sensitivity of 85% and a specificity of 100%.

When urine is used, DNA amplification methods have a sensitivity of around 90%. The sensitivity of self-collected vaginal samples analyzed by LCR is slightly better (93%) than with PCR analysis (88%). A review of the literature has shown that both methods have specificities of 99%, regardless of the specimen used, Dr. Rein said.

Cost is also a key advantage. C. trachomatis is an intracellular pathogen and so it has been traditionally diagnosed by tissue culture, which is expensive. Compared with PCR and LCR analyses, which cost $5.41 and $5.32, respectively, materials for the traditional culture test cost $22.56, he said.

The newer DNA amplification methods are also far more sensitive in terms of the number of organisms required for a positive test compared with the first culture-independent test, the enzyme-linked immunosorbent assay (ELISA). The ELISA method of analyzing cervical samples has a sensitivity of only about 70% and a specificity of 95%.

With all these advantages, it's dear that LCR and PCR assays "are very snappy tests and I think are having a great role in the management of chlamydial infection. ... One can do noninvasive screening for chlamydia, which is very attractive from the standpoint" of high-risk populations.

But, Dr. Rein added, these assays are also a two-edge sword: Their ease may also lead to inappropriate use. They're excellent screening tools, but they should not replace clinical evaluation or rule out the need to treat patients on the basis of sexual exposure.

Dr. Rein explained: In everyday clinical practice "we will diagnose nongonococcal urethritis [NGU] in a man by finding polymorphonuclear neutrophils in his penis. After treating that patient on the basis of that Gram stain finding, he'll return, usually with a recurrence of symptoms, and say, 'My girlfriend went to her doctor, tested negative for chlamydia, and was told she didn't need treatment.'

"Please bear in mind that only about 30% of nongonococcal urethritis is chlamydial. Therefore, the standard of care for managing female partners of men with NGU is treatment. Even if you're doing one of the very accurate new tests for chlamydia, you can't rule out infection in a female patient. All you can rule out is chlamydial infection."

And since the majority of cases of NGU aren't chlamydial, the absence of chlamydial infection doesn't preclude the need to treat, he said. Even though these tests are very good, "beware of applying them willy-nilly to low prevalence populations" because when they're used to screen such populations their predictive value goes way down, he said.

Assuming that the tests were 99% sensitive and 99% specific, 1 in 100 noninfected patients would have a positive test. And in a population in which only 1 in 100 people has the disease, only 50% of the positive tests would be true positives.
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Publication:OB GYN News
Date:Jun 1, 2001
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