SYPHILIS TESTING - serum or CSF

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Specimen: 5-10 mL blood in plain tube, CSF 1-5 mL in plain tube.

Method: RPR, VDRL are non-specific cardiolipin antibody tests. FTA-ABS, TPHA and TPI measure specific antibodies to Treponema pallidum antigens. Some laboratories use EIA for either non-specific or specific antibodies, as an alternative to the traditional tests.

Application:

Serum tests: Patients with suspected syphilis and contacts; antenatal screening; blood and tissue donors; patients with STD, HIV infection. RPR or VDRL are used as screening tests; FTA-ABS or TPHA are used as confirmatory tests. TPI is a confirmatory test which is now seldom used.

CSF examination: microscopy, protein estimation; VDRL and FTA-ABS are performed to exclude or support a clinical diagnosis of neurosyphilis and are used in the investigation of unexplained dementia.

Interpretation:

Serum tests: The RPR and VDRL are sensitive but non-specific tests: positive results may indicate active syphilis but confirmatory tests for specific antibody to T. pallidum are required. RPR or VDRL are also used for monitoring treatment. The titre falls with successful treatment, but these tests may not become negative unless treatment is commenced early in the course of the infection. Biological false positives may be found in pregnancy; transiently in eg measles, chicken pox; chronically in eg cirrhosis, SLE, the phospholipid antibody syndrome, leprosy.

FTA-ABS, TPHA, TPI: positive results confirm the diagnosis of syphilis, but do not indicate whether the disease is active, inactive or cured. Titres may remain elevated after effective therapy, although they may become negative if treatment has been commenced early. The titre may not fall after effective treatment, except in early syphilis.

CSF examination: In the presence of positive serum tests, the finding of one or more of an increased CSF white cell count, increased CSF protein or positive VDRL supports a clinical diagnosis of neurosyphilis. However, any or all of these CSF tests may be normal in the presence of neurosyphilis.

Reference: Larsen SA. Clin Microbiol Rev 1995; 8: 1-21.

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