Laboratory Diagnosis of Syphilis

Laboratory Diagnosis of Syphilis
Diagnosis of syphilis is based on microscopy and serology. At the first antenatal visit, all women in UK are screened for sexually transmitted diseases including syphilis and HIV. The serological tests are repeated at three monthly intervals in cases of anogenital ulceration if the initial tests are negative. All infants born to seropositive mothers should be examined at birth and at monthly intervals for 3 months until it is confirmed that serological tests are and remain negative.
Microscopy
Microscopic demonstration of T. pallidum from the lesions or infected lymph nodes in early syphilis depends on the following three tests:
Dark-field microscopy: if a lesion such as chancre is present, dark-field microscopy should be attempted to visualize the characteristic motile spirochetes in the exudates collected from the lesion. The sensitivity rate[5] is up to 97%, so failure to find the organism does not exclude a diagnosis of syphilis. (For an explanation of sensitivity and specificity, please see Lalkhen and McCluskey.[8])
Direct fluorescent antibody (DFA) test: this uses the indirect fluorescent technique with killed T. pallidum as antigen. The organisms are fixed on a slide to which serum is added. The antibody in the serum unites with treponemes and is made visible with fluorescent stain.[1]
Polymerase chain reaction (PCR) test: it may be useful for the detection of primary syphilis with sensitivity up to 98.6%.
Serological Tests
Non-treponemal Tests. These tests detect the cross-reaction of antibody to syphilis with cardiolipin. The result is reported as reactive or non-reactive; a reactive test is accompanied by a quantitative titre and should be confirmed with a treponemal test. False positive non-treponemal tests may occur in patients who are pregnant, i.v. drug users, those with systemic inflammatory diseases such as systemic lupus erythematosus, or after a recent viral infection.[3]
VDRL (venereal disease research laboratory) test: this is simple and inexpensive and is the preferred test worldwide.
RPR (rapid plasma reagin) test: this is used for screening purposes and is the least technically demanding test as no microscope is needed. It uses carbon-containing cardiolipin antigen and requires a minimal quantity of blood.
Treponemal Tests. These tests specifically detect antibodies against T. pallidum and are positive for life in the vast majority of infected patients regardless of stage or treatment history.[3]
TPHA (T. pallidum haemagglutination assay) or TPPA (T. pallidum particle agglutination assay): these are very valuable and simple tests using an indirect haemagglutination method with red cells or by gelatine particles. Together with VDRL, it is probably the best combination for routine use. False positive reactions occur in up to 2%.[1]
EIA (enzyme immuno assay): treponemal enzyme immunoassay is the screening test of choice and can detect IgG and IgM antibodies as it is positive in earlier stages of syphilis. A positive test is then confirmed with the TPHA/TPPA or VDRL/RPR tests.
FTA-ABS (fluorescent treponemal antibody absorption) assay: this uses the indirect fluorescent technique with killed T. pallidum as an antigen. The organisms are fixed on a slide to which serum is added. The antibody in the serum unites with treponemes. The test has been made more specific by absorbing the group antibodies. This is the most sensitive and specific test available. It becomes positive earlier during the initial stage of primary syphilis. However, it is not suitable for assessing the activity, as the positive test persists long after successful treatment.[1]
Neurological involvement[9] is confirmed by a positive VDRL, raised cell count (>5/mm2), and raised protein (40 mg dl–1) in the CSF obtained by lumbar puncture. Chest X-ray, electrocardiography, echocardiography, cardiac catheterization, and biopsy of gumma can reveal involvement of other systems.