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BC Public Health Microbiology Laboratory switching to an automated syphilis screening test
Jul 2, 2014 by Muhammad Morshed, Program Head, Zoonotic Diseases & Emerging Pathogens, BC Public Health Microbiology Reference Laboratory
Syphilis, caused by the bacterium Treponema pallidum sub spp pallidum, can be sexually transmitted as well as spread from an infected mother to her fetus and through blood transfusions. Globally, 25 million people are infected with syphilis, with an estimated annual incidence of 12 million cases.
In BC, syphilis infection rates are higher than the average Canadian rate and are particularly high amongst men who have sex with men (MSM).
Stages of syphilis infection
T. pallidum infection may produce disease in three stages. Entering through intact or abraded skin or mucous membranes, and multiplying at the site of entry, syphilis first causes painless ulcers (chancres) approximately 3 weeks post exposure. This is the first stage of syphilis, or primary syphilis. Without treatment, the chancre may resolve within 1 to 5 weeks.
Antibodies in the blood (against cardiolipin, a nonspecific antigen that cross-reacts well with T. pallidum antigens, and treponemal antigen from animal sources) usually do not appear until 1 to 4 weeks after the chancre. During this time, the ulcer heals and T. pallidum, if not treated, spreads systemically. This is the second stage of syphilis, or secondary syphilis. Multiple types of rashes and flu-like symptoms mimicking many other diseases may appear about 2 to 6 weeks later. This second stage, if untreated, either resolves within 2 to 6 weeks or the infection can proceed to the third stage (latent or late stages of syphilis) as long as 30 years later.
One-third of untreated patients with third stage infections end up with chronic manifestations of disease, including gummas (in any tissue) and cardiovascular or neurological signs and symptoms.
The laboratory diagnosis of syphilis infection is complex. Since this organism cannot be cultured, serology is the mainstay for diagnosis.
Traditionally, syphilis screening tests were performed with non-treponemal antigens using the Rapid Plasma Reagin (RPR Test or the Venereal Disease Research Laboratory (VDRL) test, and if reactive, were confirmed by treponemal tests such as Treponema pallidum Particle Agglutination Test (TPPA) or FTA Abs.
Recently, due to the need to find efficiencies for high-volume screening and address ergonomic issues such as pipetting, and with the development of better tests, laboratories now have the opportunity to change their diagnostic approach.
Implementation of the new test
The BC Public Health Microbiology Reference Laboratory (PHMRL) recently evaluated and validated a Siemens ADVIA Centaur Syphilis chemiluminescence immunoassay. This fully-automated, antigen sandwich immunoassay (ADVIA Centaur XP system platform) uses direct chemiluminometric technology.
Both characterized and routine samples used in our validation studies yielded excellent sensitivity, specificity and reproducibility. Discordant samples were resolved completely using the algorithm developed. Known as the reverse algorithm, screen reactive samples are then tested with several specialized confirmatory, quantitative, non-treponemal tests (including RPR or VDRL, etc.). This current algorithm may allow the diagnosis of more early and late latent cases.
Syphilis screening using chemiluminescence based immunoassay will be implemented on July 9th 2014 in BCPHMRL’s automated High Volume Serology Program in Central Processing & Receiving (Lane Level) Laboratory. One SST blood collection tube will be required to perform this test.
Since screening tests will be done using treponemal antigens, physicians will no longer need to order confirmatory testing separately as was previously done for patients with conditions such as uveitis, neurosyphilis, congenital infections or primary syphilis.
For further information
For any diagnostic issues related to syphilis, please contact Dr. Muhammad Morshed at 604-707-2622.
For information regarding the treatment and follow-up of prenatal or congenital syphilis positive cases, please contact the Provincial STI Clinic Physician at 604-707-5606.
Categories: Current practice