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How is a Chlamydia Screening Lab Test Done? - Theory and Practice of C.Trachomatis Detection

written by: niknak•edited by: Emma Lloyd•updated: 7/13/2010

How is a chlamydia screning lab test done? There are numerous methods of detecting C. trachomatis ('chlamydia') including cell culture, enzyme-immuno assay, direct fluorescent-antibody assay and the more recently developed nucleic acid amplification tests.

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    Chlamydia Facts

    • Chlamydia trachomatis is one of the most common sexually transmitted diseases. The majority of infected people are asymptomatic and so often undiagnosed.
    • Symptoms can include a genital discharge. During examination, it may be apparent that cells of the cervix are inflamed.
    • Reported rates of genital chlamydia infections are rising, but this could be due to due increased testing.
    • Chlamydia is a bacterial infection and can be easily treated with antibiotics.
    • Infection does not always cause serious results, but has been associated with a wide spectrum of complications. Infected men may develop epididymitis, which can cause infertility. In women, infection may cause pelvic inflammatory disease which carries an associated risk of ectopic pregnancy and infertility. Sporadic and recurrent miscarriage, premature labour and low birthweight have also been linked to chlamydia infection.
    • Chlamydia transmission from mothers to babies can occur, causing conjunctivitis, nasopharyngitis and pneumonia in newborns.
    • Screening programmes have been introduced in many countries with the aim of early detection and treatment of undiagnosed infections. One of the main targets of screening programmes are young people who are sexually active.
    • Teenages are often apprehensive about being tested and will ask 'how is a chlamydia screening lab test done'. The answer is that it is painless, being done by urine analysis (men and women) or a swab (women). Swabs can be collected by the patient themselves, mimimizing anxiety and embarrassment.
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    How is a Chlamydia Screening Lab Test Done?

    Isolation of Chlamydia in Cell Culture: this is 100% specific, but it has disadvantages, being laborious, expensive and relatively insensitive compared with newer test methods. Another problem is that clinical material has to be handled in a specific way e.g. time and storage conditions, for the test to be accurate.

    Antigen Detection: The use of monoclonal antibodies makes the direct detection of chlamydia in clinical specimens possible. The target antigens that are are DFA and EIA. The sensitivity of DFA is 80–90% and the specificity is 98–99% compared to cell culture. 'Rapid tests' which do not require sophisticated equipment and can be completed quickly, often employ EIA technology. Although less sensitive and specific than laboratory EIA and DFA tests, they have been useful in the detection of C. trachomatis infections of the eye.

    DNA and RNA Detection: Chlamydia can be identified by detecting its nucleic acids (DNA or RNA). Initially, direct detection of nucleic acid was performed without its amplification. Newer techniques amplify the nucleic acid, making the detection of very low amounts of DNA or RNA possible. These tests are therefore far more sensitive than prevous tests and show comparable sensitivities for male and female urine, urethral and cervical swab specimens.

    Antibody Detection in Serum: Different serological assays have been developed for the detection of antibodies to C. trachomatis, including the complement fixation test, micro-immunofluorescence assay, EIA and immunoblotting. Superficial infections stimulate poor antibody responses, however, a correlation between antibody levels and the severity of inflammation has been shown. IgG antibodies persist for years even after antibiotic treatment. Chlamydia IgG antibody testing in serum is often used in patients undergoing treatment for infertility, but it has no value in early diagnosis and national screening programmes.

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    References

    Epidemiology of Chlamydia trachomatis infection in women and the cost-effectiveness of screening by J.Land, J.Van Bergen, S.Morré & M.Postma. Human Reproduction Update 2010, Vol 16, P189-204

    UK NHS National Chlamydia Screening Programme: www.chlamydiascreening.nhs.uk