S
yphilis is a sexually transmitted disease caused by the spirochetal
bacterium Treponema pallidum variant pallidum. After reaching
a 60-year nadir in 2000, cases of primary and secondary syphilis in the
United States have increased yearly, including an 18% rise between 2007
(N=11,466) and 2008 (N=13,500). As incidence of syphilis increases, syphilis
testing remains a cornerstone of prevention and control efforts. What has
begun to change, however, is the availability of new types of syphilis tests
and laboratory testing algorithms. This article describes types of serologic
tests for syphilis that are performed on sera, discusses traditional and
newer algorithms for use of those tests, and summarizes data on use of newer
algorithms.
Two types of tests
The two types of serologic tests for syphilis are
non-treponemal tests and treponemal tests. Commonly used nontreponemal tests
include the rapid plasma reagin (RPR) test and the Venereal Disease Research
Laboratory (VDRL) test. Both of these tests detect antibodies to cardiolipin,
which are non-specific markers for syphilis. Non-treponemal tests
typically become reactive within several weeks following infection,
remain reactive without treatment, and can be persistently reactive for
months to years following treatment. Importantly, biologic false positive
(BFP) results of non-treponemal tests can occur in the setting of older age,
autoimmune disease, intravenous drug use, recent vaccination, or certain
infections. In addition to qualitative results, non-treponemal
tests can be quantitated by serially diluting the serum until the test is no
longer reactive. The ratio of serum to diluent of the most dilute serum at
which the test is still reactive is called the titer. Non-treponemal test
titers correlate with, and are used as a marker of, disease activity and
response to treatment.4
As clinical laboratories continue to use or to switch over to
newer algorithms for syphilis testing, laboratorians and clinicians will
have to become more comfortable with interpreting test results produced
when starting with a treponemal EIA test.
Treponemal tests, by contrast, detect antibodies
specific to T pallidum and other pathogenic treponemal species.3
Commonly used treponemal assays include non-enzyme immunoassays, such as the
T pallidum particle agglutination (TP-PA) test, the T pallidum
hemagglutinin assay (TPHA) and the fluorescent treponemal
antibody-absorption (FTA-ABS) test, and treponemal enzyme immunoassays and
immunochemoluminescence (EIA) tests (hereafter collectively referred to as
treponemal EIA tests). Treponemal tests typically become reactive within
several weeks following infection (sometimes preceding reactivity of non-treponemal
tests) and, in most cases, remain reactive for life. Unlike non-treponemal
test titers, treponemal test titers do not correlate with disease activity
or response to treatment.4
The syphilis-testing algorithm in the United States
has traditionally started with a non-treponemal test, because those tests
have historically been less expensive for laboratories to perform than
treponemal tests.2,6 According to the traditional algorithm (see
Figure 1), a non-reactive non-treponemal ends the testing process (except
when very recent infection is suspected, in which case clinicians sometimes
request that the laboratory perform a treponemal test, which can become
reactive before a non-treponemal test does). A reactive non-treponemal test
leads to titer quantitation and then to confirmatory testing with a
treponemal test. A reactive treponemal test confirms an old or new diagnosis
of syphilis (or other treponemal disease, which is rare in the United
States). A non-reactive treponemal test suggests that the reactive non-treponemal
test was a BFP.
Figure 1. Traditional laboratory
testing algorithm for syphilis.
Testing algorithms change
Newer syphilis testing algorithms, by contrast,
reverse the order of use of non-treponemal and treponemal tests (see Figure
2). The newer algorithms capitalize on the fact that treponemal EIA tests
can be automated, making it economically more efficient in settings of high
testing volumes to start the testing algorithm with a treponemal EIA test,
rather than a non-treponemal test. In the newer algorithm, testing concludes
if the treponemal EIA test is non-reactive. A reactive treponemal EIA test
leads to testing with a non-treponemal test and titer quantitation which, in
some newer algorithms, leads to further testing if the non-treponemal test
is non-reactive.
Figure 2. Newer laboratory testing
algorithm for syphilis.
Importantly, all of the newer algorithms can produce
a combination of results (i.e., a reactive treponemal test and a
non-reactive non-treponemal result) that is not produced with the
traditional algorithm (unless a clinician specifically requests a TPPA
despite a nonreactive non-treponemal test). Variations of laboratory testing
and reporting practices as well as limited clinical experience with patients
with results not typically seen with the traditional algorithm can
complicate interpretation of results produced by newer algorithms.7
A study published in the Centers for Disease Control
and Prevention’s MMWR on Aug. 15, 2008, investigated use of the newer
screening algorithm and highlighted some of the variations in laboratory
practice and complications of interpretation that occur in some cases.7
The study investigators reviewed data on a convenience sample of 116,822
specimens tested during Oct. 1, 2005 to Dec. 1, 2006, at four New York City
laboratories that used syphilis screening algorithms that started with a
treponemal EIA. (Specifics of the algorithm used by each laboratory after a
reactive treponemal EIA test are available in the MMWR article.)
Composite study results and their recommended interpretations were as
follows:
- Of all 116,822 specimens tested, 6% were reactive to a treponemal
EIA test, of which 99% were then tested with the RPR test. Further
testing was not performed on the 94% of specimens initially nonreactive
to the treponemal EIA test. Patients whose specimens were non-reactive
to the treponemal EIA test either do not have syphilis or have very
recent infections. - Of all 6,548 specimens reactive to the treponemal EIA test and then
tested using an RPR test, 44% were reactive and 56% (3% of the overall
sample) non-reactive with an RPR test. Patients whose specimens were
reactive to both treponemal EIA and RPR tests should be considered to
have an old or new case of syphilis. If previous treatment can be
documented and the titer has not increased at least fourfold, those
patients require no further management. Re-treatment should be provided
if a previously treated patient has a fourfold or greater increase in
titer. Patients whose specimens are reactive to a treponemal EIA test
but non-reactive to a non-treponemal test have old, previously treated
syphilis; very recently acquired syphilis; or a false-positive
treponemal EIA test result. For those patients, performing a different
treponemal test (e.g., TP-PA, TPHA, or FTA-ABS) can help assess whether
the treponemal EIA test result was a false positive. - Of all 3,664 specimens reactive to the treponemal EIA test and
non-reactive with the RPR test, 69% were tested with a different
treponemal test. Of those specimens tested with a different treponemal
test, 83% were reactive with the different treponemal test. Patients
with those test results should be staged and treated, unless previous
treatment can be documented. The other 17% of patients were non-reactive
(e.g., potentially had false- positive results) with a different
treponemal test. For those patients, clinicians may decide to forego
treatment, or a third treponemal test can be used to help resolve the
discrepancy. Clinicians should also incorporate individual patients’
histories and physical findings in interpreting syphilis testing
results.
As clinical laboratories continue to use or to switch
over to newer algorithms for syphilis testing, laboratorians and clinicians
will have to become more comfortable with interpreting test results produced
when starting with a treponemal EIA test. Additional clinical experience
with patients diagnosed and treated using the newer testing algorithms
should contribute to that goal — and to the overall aim of improved syphilis
prevention and control.
Kenneth Katz, MD, MSc, MSCE, is STD Control Officer
and Senior Physician of the HIV, STD and Hepatitis Branch, Health and Human
Services Agency for the County of San Diego, CA.
Acknowledgments: The author would like
to thank Elaine Pierce, MD, MPH; Jeffrey Klausner, MD, MPH; Julia Marcus,
MPH; and Ina Park, MD, MS, for helpful comments in preparing the manuscript.
References
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Infectious syphilis in high-income settings in the 21st century. Lancet
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