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sexually transmissible diseases
The diagnostic laboratory uses serological tests to investigate patients suspected of suffering from syphilis. This disease is caused by the spirochete Treponema pallidum, resulting in a chronic infection with many diverse clinical manifestations that occur in three distinct stages. The organism cannot be readily cultured and hence diagnosis relies heavily on serological techniques. Sera are screened by a combined IgG and IgM ELISA test with any reacting sera being tested by various confirmatory tests. All specimens are tested by the ELISA screening test for IgM and IgG antibodies. CSF samples and specimens sent for confirmatory testing are routinely examined using the Treponema pallidum particle agglutination (TPPA) test and the rapid plasma reagin (RPR) tests in addition to the ELISA screen.
During the primary stage of syphilis infection antibodies to a number of Treponema pallidum antigens are discernable in the blood, sometimes before and often concurrent with or slightly after the appearance of the first clinical symptoms. Antibodies to several additional antigens are detectable during the secondary and early latent stages of syphilis and responses to particular antigens are evident throughout the entire course of the disease. Early IgM and later IgG responses to Treponema pallidum infection are directed primarily against the same set of antigens. After treatment and during later stages of the disease the IgM reactivity to many antigens attenuates; the IgG response remains. Patient sera are now routinely screened for these antibodies using an ELISA test.
The Treponema pallidum haemagglutination (TPHA) test is a specific test for antibodies to treponemal antigens. It relies upon the use of treponemal antigens that are attached to avian red blood cells. Although very useful for many years, this test has been superseded by the TPPA test. In the TPPA test, stained gelatin particles coated with Treponema pallidum antigens substitute for the coated avian red blood cells of the old TPHA test. The TPPA test is used to confirm the results of the ELISA screening test.
The RPR test employs an antigen that is not of treponemal origin. Because of tissue damage that occurs in a syphilitic lesion, circulating antibodies, termed "reagin antibodies", are produced against tissue components. The RPR antigen is a colloidal suspension of a cardiolipin-cholesterol-lecithin complex that reacts immunologically with reagin to form visible floccules. The visual difference between a positive and a negative reaction is enhanced by microparticulate carbon in RPR Carbon Antigen, which contains carbon particles. This test replaces the older venereal disease reference laboratry (VDRL) test, which works on the same principle.
Additionally the fluorescent treponemal antibody test (FTA abs) may be employed. This is a very useful test to perform to confirm an unexpected positive TPPA result. In the FTA abs test, treponemes are fixed to a microscope slide, to which the patient’s serum is added. If this is positive, antibodies will attach to the Treponema pallidum antigens. The serum is then washed off, and the slide treated with a fluorescent labelled antihuman antibody serum. In a positive test this will attach to the patient’s antibodies, and may be detected using fluorescence microscopy.
Once positive, both the TPPA and FTA abs tests remain so for life, as a marker for syphilis. The RPR test becomes negative after successful therapy, and can be used to monitor treatment. However, because of the non specific nature of the RPR test, occasional biological false positives occur in patients with underlying disease processes other than syphilis.
Urethral and high vaginal swabs are plated on VPAT agar incubated under 5 10% CO2 and a Sabouraud's agar plate incubated aerobically. VPAT is used to select for Neisseria gonorrhoeae, and Sabouraud's agar is used to culture yeasts. A wet preparation is examined for Trichomonas vaginalis, and a Gram film is examined.
An aerobic fresh blood agar plate and a MacConkey plate, together with a neomycin fresh blood agar plate incubated anaerobically are also included in the regime. If the patient has suspected pelvic inflammatory disease, a chocolate agar plate is incubated under 5 10% CO2, and the anaerobic culture is on fresh blood agar and is maintained for 5 days to aid the detection of Actinomyces israelii. In the investigation of sexually transmissible diseases, throat and rectal swabs may be plated onto VCAT agar and incubated under CO2 to detect Neisseria gonorrhoeae.
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