Syphilis Antibody, IgG, Serum
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Syphilis is a disease caused by infection with the spirochete Treponema pallidum. The infection is systemic and the disease is characterized by periods of latency. These features, together with the fact that Treponema pallidum cannot be isolated in culture, mean that serologic techniques play a major role in the diagnosis and follow-up of treatment for syphilis.
Historically, the serologic testing algorithm for syphilis included an initial nontreponemal screening test, such as the rapid plasma reagin (RPR) or the venereal disease research laboratory (VDRL) tests. Because these tests measure the host's antibody response to nontreponemal antigens, they lack specificity. Therefore, a positive result by RPR or VDRL requires confirmation by a treponemal-specific test, such as the fluorescent treponemal antibody-absorbed (FTA-ABS) or microhemagglutination assay (MHA-TP). Although the FTA-ABS and MHA-TP are technically simple to perform, they are labor intensive and require subjective interpretation by testing personnel.
Recently, EIA and multiplex flow immunoassays (MFI) were introduced to assess serologic response to Treponema pallidum. The Bio-Rad BioPlex Syphilis IgG assay is an example of MFI technology, which utilizes specific, treponemal antigens coated on microspheres for the detection of IgG-class antibodies to Treponema pallidum. The BioPlex Syphilis IgG assay is highly sensitive and specific (see Supportive Data), and allows for an objective interpretation of results. Due to several factors including the low prevalence of syphilis in the United States, the increased specificity of treponemal assays, and the objective interpretation of MFI and EIA technology, initial serologic testing by a treponemal-specific assay (eg, EIA or MFI) is now commonly performed in clinical laboratories. Specimens testing positive by the treponemal-specific assay are then tested by RPR to provide supplementary serologic data, as well as to provide an indication of the patient's disease state and history of treatment.
During early primary syphilis, the first antibodies to appear are of the IgM-class, with IgG-class antibodies reaching significant titers later in the primary phase. As the disease progresses into the secondary phase, IgG-class antibodies to Treponema pallidum reach peak titers, and may persist indefinitely regardless of the disease state or prior therapy.
For prenatal syphilis screening, the IgG test is recommended. IgM testing should not be performed during routine pregnancy screening unless clinically indicated.
Treponema pallidum IgG antibodies persist indefinitely, regardless of the course of the disease. If treatment of an original Treponema pallidum infection was not monitored, a diagnosis of reinfection may actually represent either a resurgence of an inadequately treated earlier infection or persistent IgG antibodies from a resolved infection.
An aid in the diagnosis of active Treponema pallidum infection
Interpretive results for syphilis IgG:
1. IgG positive and RPR positive: active or recently treated syphilis
2. IgG positive and RPR negative: past, successfully treated, or latent syphilis
Infants <6 months demonstrating antibody to IgG or a positive response to RPR have probable maternal antibody. Testing for
IgM antibody (Misc Referral #90522: Focus Diagnostics, unit code #20493 Treponema pallidum IgG and IgM Antibody
Panel, IFA) should be performed.
RPR results may be useful for determining the current disease status and response to therapy. Response to treatment may be indicated by a decrease in RPR titers or the reversion of a positive to negative result for the IgM assay. Because reactive treponemal IgG antibody test results usually remain reactive for a lifetime, IgG antibody levels should not be used to determine response to therapy.
3. IgG negative (RPR not performed): No evidence of active syphilis (see Cautions)
Patients suspected of having very early syphilis may be negative for IgG to Treponema pallidum and should be retested approximately 1 week later.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Despite active syphilis, serologic tests may be negative in severely immunosuppressed patients such as those with AIDS.
In very early cases of primary syphilis, both IgM and IgG serologies may be negative.
In cases of old, successfully treated infection (>10 years earlier), both IgG and IgM serologies may be negative.
Results should be considered in the context of all available clinical and laboratory data.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Tramont EC: Treponema pallidum (Syphilis). In Principles and Practice of Infectious Diseases. Fifth edition. Edited by GL Mandell,
JE Bennet, R Dolin. New York, Churchill Livingston, 2000, pp 2474-2491