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Syphilis Serology

The causative organism of syphilis, Treponema pallidum, cannot be cultured in the clinical microbiology lab. Therefore, serologic tests are necessary for the diagnosis of the disease in any stage. Serologic tests for syphilis are classified as either nontreponemal or treponemal.

Nontreponemal tests include RPR and VDRL. These tests detect IgM and IgG anti-cardiolipin antibody, which is produced in response to host cell damage and cardiolipins released from treponemes. Consequently, false positive nontreponemal tests occur in the presence of other anti-cardiolipin antibody-producing conditions including autoimmune disease, various other infections, pregnancy, and previous transfusions. All reactive nontreponemal tests should be followed up with treponemal testing. Titers of reactive nontreponemal tests can be monitored for effectiveness of therapy—a fourfold decrease in titer indicates therapeutic response.

RPR titers are negative or low early in primary syphilis.  As the disease progresses, titers increase and a greater percentage of patients develop a reactive RPR.  RPR titers may decrease after several years, even without therapy.  RPR titers drop rapidly after treatment of primary or secondary syphilis. RPR can be used to monitor response to therapy Most patients become seronegative at 16 to 18 months after initiation of treatment. Approximately 30 percent of patients remain weakly seropositive.

RPR is not highly specific. Biological false positive reactions (BFP) occur in about 1 in 4,000 persons in the general population and 1 in 2,000 pregnant women.  Both acute and chronic biologic false positive reactions occur.  Viral and bacterial infections, immunizations, and pregnancy may cause acute BFP.  The titers are usually low and decrease with time.  Autoimmune diseases, senescence, cirrhosis, metastatic cancer, lymphomas, myeloma, drug abuse, and anti-cardiolipin antibody syndromes cause chronic BFP.  Reactivity usually persists more than six months and the titer remains fixed at a low level.

Nontreponemal assays have low sensitivity in primary syphilis. RPR sensitivity is 62 to 76 percent and VDRL sensitivity is 62 to 78 percent. They also have low sensitivity in latent syphilis. RPR sensitivity is 61 percent and VDRL sensitivity is 64 percent.

Treponemal tests include FTA-ABS and TP-PA. The FTA-ABS test measures treponemal antibodies by indirect fluorescence, while TP-PA is a particle agglutination test that has widely replaced MHA-TP. TP-PA sensitivity is 86 to 100 percent in primary syphilis and 91 to 98 percent in latent syphilis. Treponemal tests become reactive earlier in primary syphilis than the RPR and remains reactive in greater than 90 percent of patients with latent syphilis, irrespective of treatment.  Therefore, treponemal tests cannot be used to monitor response to therapy.

Treponemal tests can be falsely positive due to Epstein Barr virus infections, Lyme disease, and autoimmune disease. A reactive treponemal test in addition to a reactive nontreponemal test is highly specific for infection.

Clinically, syphilis has four defined stages. The primary stage is defined by the presence of a chancre at the site of infection. In secondary syphilis, the organism disseminates to multiple sites, causing rashes & lymphadenopathy. Primary and secondary syphilis generally occur in the first 6 months of untreated infection. Latent syphilis is asymptomatic infection, occurring after or between primary & secondary stages. Tertiary syphilis consists of late complications of the disease, including aortic aneurysm and neurosyphilis, usually occurring years or decades after primary infection. Serologic tests have variable sensitivity at each stage of disease. The % sensitivity of each serologic test by stage of infection is:

	Primary	Secondary	Latent	Tertiary
VDRL	62-78	100	61	71
RPR	62-76	100	64	73
FTA-ABS	84	100	91-98	96
TP-PA	86-100	100	91-98	96
Syphilis IgG	97	97	97	97

 

The specificity for all tests in all stages of infection is 96-99%. Of note, nontreponemal tests have low sensitivity (70%) in tertiary syphilis. Therefore, treponemal tests should be performed regardless of RPR/VDRL results when late complications of the infection, such as aortic aneurysm or neurosyphilis, are suspected.

Traditionally, laboratories have screened serum samples using a non-treponemal test and only used a treponemal assay to confirm screen-positive samples. This algorithm was orginally recommended by CDC due in part to the fact that non-treponemal tests are inexpensive, relatively easy to perform, show good correlation with disease status, and may be used to follow a patient’s response to therapy. Despite these advantages, non-treponemal tests are subjective and non-specific, and may result in false-positive results, especially in areas with a low prevalence for the disease. Furthermore, tests such as RPR require manual processing—a significant limitation for clinical laboratories testing a high volume of samples.

For these reasons, many clinical laboratories have adopted a reverse sequence screening algorithm that begins with a treponemal assay, such as EIA or MFI, and then confirm screen-positive samples with an RPR. If RPR is positive, the patient can be diagnosed with syphilis. Samples that are positive by the screening test but negative by RPR should then be tested by a second treponemal assay for confirmation. A positive treponemal test and negative RPR could mean that the treponemal screen was a false positive or that the patient had a past infection that was treated, or the patient could have primary or latent syphilis. Most often, the second treponemal assay is TP-PA. If it is negative, the first treponemal test can be considered a false positive.

From a clinical perspective, the reverse algorithm is preferable because of its increased sensitivity in primary and latent disease compared to the traditional algorithm. However, up to 17 percent of the treponemal positive results do not confirm with the RPR and require a second treponemal assay.

Reverse Syphilis Serology Interpretation Guidelines
Screening EIA (Treponemal)	RPR (Non-Treponemal)	TPPA (Treponemal)	Interpretation
Nonreactive	N/A	N/A	Negative for syphilis. No further testing required, unless clinically indicated.
Reactive	Reactive	N/A	Consistent with untreated or recently treated syphilis. See CDC treatment guidelines.
Reactive	Nonreactive	Reactive	Consistent with past successfully treated syphilis.  Early or latent syphilis possible in individuals with no history of treated syphilis.
Reactive	Nonreactive	Nonreactive	Consistent with false positive screening test. No further testing required, unless clinically indicated.

 

VDRL testing on CSF for the diagnosis of neurosyphilis is highly specific (99.8%), but has low sensitivity (50%), therefore a reactive CSF VDRL is diagnostic, but a non-reactive result does not rule out neurosyphilis. FTA-ABS can also be performed on CSF, and has 100% sensitivity, but lower specificity than VDRL (94%).