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Chlamydia trachomatis infection is the most common sexually transmitted disease in the United States and Europe. Chlamydia causes an estimated four to five million infections each year in the U.S. Men, women, and infants are affected, but women are at greatest risk of serious complications. In women, Chlamydia infections begin as a silent infection of the cervix, which can last up to 21 days. Ascending disease involves the fallopian tubes and ovaries, producing pelvic inflammatory disease. Chlamydia infections are a leading cause of infertility and ectopic pregnancy. About 70% of infected mothers transmit Chlamydia to the newborn during birth. Infected infants are at increased risk of developing pneumonia and conjunctivitis. In men, Chlamydia causes approximately one-half of the cases of non-gonococcal urethritis. Chlamydial urethritis is 2.5 times more common than gonococcal urethritis. In addition, it causes about half of the estimated annual 500,000 cases of acute epididymitis. Chlamydial proctitis and pharyngitis occur in homosexual males. Because of the increasing incidence of Chlamydial infections and their serious sequelae, screening has become increasingly important. The U.S. Preventive Services Task Force issued updated guidelines in 2001 for Chlamydia trachomatis infection screening (www.ahcpr.gov/clinic/uspstf/uspschlm.htm). The recommendations state that primary care physicians should routinely screen all sexually active women under the age of 25. According to data from the CDC, almost half of all reported infections are in 15 to 19 year old women, with women aged 20 to 24 accounting for an additional 33 percent of cases. Screening should also be done, regardless of age, in women who have more than one sexual partner, have had a previous STD, or who do not use condoms consistently and correctly. No suggestions were made regarding screening of asymptomatic men or women over 25 with no other risk factors. Based on review of studies by the Task Force, screening patients at greatest risk is more cost effective than screening all patients, and the cost of screening is less than treating the infection and its complications. Recent studies also show that many women at risk for infection are not being screened. One method for performing Chlamydia detection is the Roche Cobas Amplicor PCR for CT/NG testing. Performance characteristics of this assay include a sensitivity of 97% and specificity of 99% from endocervical swabs. Positive predictive value (PPV) is significantly impacted by the prevalence of disease in the population being tested and is approximately 90%. A recent publication by the CDC recommends that all positive screening tests for CT and NG should be considered presumptive evidence of infection. Additional testing should be considered after a positive screening test when the PPV is < 90%, or when there are significant adverse social or psychological consequences from a positive screening result (MMWR 2002; 51,No.RR-15). Recently, SLRL added the following comment to all positive results, "False positive results should be considered when patients have an unexpected positive screening test for Chlamydia trachomatis or Neisseria gonorrhoeae, especially when clinical findings are not supportive." In June 2005, clinicians who perform Pap smears may have received information from commercial laboratories promoting testing for Chlamydia trachomatis and Neisseria gonorrhoeae (CT/NG) from the same specimen collected for ThinPrep Pap smears. ThinPrep cytology instruments were designed to safely and effectively handle cells, not DNA (Warde Report 2003;14:1-3). Although a separate filter is used with each specimen, the fluid in the ThinPrep vial is manipulated by air pressure. ThinPrep processing can generate DNA-packed aerosols that contaminate subsequent specimens. In contrast, molecular diagnostics laboratories are familiar with the potential for aerosol contamination and utilize specific design characteristics, laboratory practices, and quality control to minimize the possibility of erroneous results. These aerosol control features are not incorporated into the ThinPrep cytology instrument. Aerosol generation is not a trivial issue because the transfer of just one nucleic acid molecule can produce a false-positive CT/NG result. The use of ThinPrep vials for CT/NG may also produce false-negative results due to specimen dilution. Dilution of the sample in ThinPrep vial fluid is of particular concern when screening asymptomatic women, as these patients often have lesser amounts of the infecting organism to be detected. The PCR Chlamydia/GC test is approved for male and female specimens including urine and endocervical or urethral swabs. A separate specimen should be collected when CT/NG testing is required concurrently with the Pap smear. Swab specimens should be submitted in M4 transport media and refrigerated. Transport media may be obtained from SLRL. Urine specimens should consist of the first 10-50 mL of the urine stream. Urine should be submitted in a clean container without preservatives, and may be transported at room temperature within 24 hours of collection. Excessively bloody specimens may cause false positive. Excessive mucus in endocervical samples may cause false negative results. Results are reported as positive or negative for Chlamydia by PCR. |
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