Treponema pallidum - Syphilis

The Treponema pallidum subspecies pallidum is the etiologic agent of syphilis. Syphilis is usually transmitted sexually, but can also be passed vertically from mother to child either in utero (congenital syphilis) or perinatally during birth.    Regardless of the mode of transmission, untreated cases of syphilis can result in multisystem involvement with significant morbidity. The disease is systemic and is characterized by periods of latency. Serologic testing is the preferred method of diagnosis. Two types of serologic tests are used: treponemal and nontreponemal assays. Traditional serologic screening for syphilis begins with a nontreponemal test followed by a treponemal test to confirm reactive results. Reverse screening algorithms begin with a treponemal test followed by a nontreponemal test to confirm reactive results.

Quick Answers for Clinicians

Which laboratory tests are used to diagnose syphilis?

In the United States, two broad categories of serologic tests, treponemal and nontreponemal tests, are used in combination according to one of the two existing algorithms (traditional and reverse) to diagnose syphilis. Diagnosis requires the use of both types of tests. Either type of test can be used as the initial screening test or the confirmatory test; however, within the treponemal and nontreponemal test categories, certain tests are recommended for screening versus confirmation. Microscopic darkfield examination, a complex direct detection method, is unavailable in most laboratories. Syphilis can be detected by polymerase chain reaction (PCR), but clinical diagnostic PCR assays cleared by the U.S. Food and Drug Administration (FDA) are not yet available.  Additionally, Treponema pallidum cannot be cultured in most laboratories and cannot be viewed on a Gram stain. 

What is the difference between the two diagnostic approaches to syphilis (traditional versus reverse screening)?

Serologic testing for syphilis diagnosis is based on an algorithm. Two categories of serologic tests are used to diagnose syphilis: treponemal and nontreponemal tests. One of two algorithms is currently recommended for testing: the traditional or the reverse algorithm. In the traditional algorithm, the screening assay is a nontreponemal test that must be confirmed with a treponemal test. In the reverse algorithm, the screening assay is a treponemal test (usually automated chemiluminescence immunoassay [CIA]) followed by a nontreponemal test. Discordant results in the reverse algorithm should be followed by a different treponemal test.  There are advantages and disadvantages to both approaches; refer to the Comparison of Nontreponemal and Treponemal Serology Testing table for more information.

How is congenital syphilis diagnosed?

The initial evaluation for congenital syphilis in infants and children should include a quantitative Venereal Disease Research Laboratory (VDRL) or rapid plasma reagin (RPR) titer; the test should be the same as that which was performed in the mother so that the titers can be compared. A serum VDRL or RPR titer that is fourfold higher than the corresponding maternal titer is considered serologic evidence that supports a diagnosis of congenital syphilis.  

Which testing algorithms are related to this topic?

Indications for Testing

Persons with signs or symptoms of syphilis infection should be tested. Additionally, asymptomatic individuals at high risk for syphilis (or of transmitting the disease to others) should be screened for infection. This includes pregnant women, individuals with a sexual partner who has syphilis, sexually active men who have sex with men, and persons infected with HIV.

Laboratory Testing

Diagnosis

Serologic testing is key to diagnosing syphilis, an infection caused by the T. pallidum spirochete. Two categories of serologic tests are used to diagnose syphilis: treponemal and nontreponemal tests. Diagnostic, serologic testing for syphilis is based on an algorithm. One of two algorithms is currently recommended for testing: the traditional or the reverse algorithm. In the traditional algorithm, the screening assay is a nontreponemal test that must be confirmed with a treponemal test. In the reverse algorithm, the screening assay is a treponemal test (usually automated chemiluminescence immunoassay [CIA]) followed by a nontreponemal test.

Nontreponemal Antibody Tests

Nontreponemal tests detect nonspecific, antilipid (reagin) antibodies that form in response to cellular damage as a result of infection.  These tests are either qualitative and reported as reactive or nonreactive, or quantitative and reported with an antibody titer. Antibody titers are important for a number of clinical scenarios, including monitoring response to therapy.  Because nontreponemal tests are not specific for syphilis, false-positive results can be seen in patients with other conditions that lead to cell damage, such as autoimmune diseases. Positive nontreponemal results should be followed with a treponemal assay, such as the T. pallidum particle agglutination (TP-PA) assay, for confirmation.  

Nontreponemal tests include rapid plasma reagin (RPR) and Venereal Disease Research Laboratory (VDRL) tests. Both RPR and VDRL can be qualitative or quantitative. The VDRL can be performed on serum or cerebrospinal fluid (CSF). VDRL performed on CSF is recommended for diagnosing neurosyphilis.

Treponemal Antibody Tests

Treponemal tests detect antibodies that specifically target T. pallidum; other conditions are unlikely to cause a positive result.  These tests are either qualitative/semiquantitative (automated CIA assays or enzyme-linked immunosorbent assays [ELISAs]) or qualitative (fluorescent treponemal antibody absorption [FTA-ABS] or TP-PA assays). Enzyme immunoassays (EIAs) and CIAs are typically available on automated platforms and used as screening assays in the reverse algorithm. The more conventional treponemal assays, FTA-ABS and TP-PA, are primarily used for confirmation in the traditional algorithm or resolution testing in the reverse algorithm. Treponemal antibodies often appear earlier than nontreponemal antibodies and usually remain detectable for life, even after treatment.   Therefore, a positive treponemal screening result must be followed by a nontreponemal test to discriminate between an active and past infection. Any discordant results between the treponemal and nontreponemal test results in the reverse algorithm must be resolved with another treponemal test, such as the TP-PA.

Comparison of Nontreponemal and Treponemal Serology Testing Approaches
  Traditional Algorithm Nontraditional Algorithm
Initial testing

Begins with a nontreponemal antibody test

Positive results must be confirmed by a treponemal test

Begins with a treponemal antibody test

Positive results must be followed by a nontreponemal test

Algorithm uses

Used to detect active infection

Quantitative tests that include antibody titers are used to monitor response to therapy

Quantitative tests that include antibody titers aid in the diagnosis of congenital syphilis

Cannot be used to establish active infection

Cannot be used to monitor response to therapy

Algorithm advantages

Detects active infection

Nontreponemal testing followed by confirmatory treponemal testing results in a high PPV

Tests are inexpensive and rapid

Detects early primary and treated infection that might be missed with traditional algorithm

Involves automated testing and low cost if used in high-volume settings

No false-negative results due to prozone reaction

Algorithm disadvantages

Nontreponemal antibody reactivity declines over time

Approach often misses early primary or treated infection

Moderately high rate of false-positive results for initial nontreponemal test (reactive test requires confirmation with treponemal test)

Treponemal antibody reactivity persists over a lifetime

Approach cannot detect active vs previously treated infection

Confirmation of reactive test result is required to detect active infection

Highly sensitive, but not highly specific

  • Frequent false-positive test results (initial reactive treponemal with negative confirmatory nontreponemal test) occur in low-risk populations and require confirmation with second treponemal test
Testing methodologies RPR, VDRL EIA, CIA, TP-PA, FTA-ABS

PPV, positive predictive value

Sources: Workowski, 2015 ; CDC,2017 ; Rhoads, 2017 

Congenital and Perinatal Syphilis

Congenital syphilis is contracted from an infected mother via transplacental transmission of T. pallidum, which can occur at any time during pregnancy; syphilis can also be acquired at birth from contact with maternal lesions. Intrauterine syphilis infection can result in stillbirth, hydrops fetalis, or preterm birth, or infected infants may be asymptomatic at birth.  

Diagnosing congenital syphilis can be challenging because maternal nontreponemal and treponemal immunoglobulin G (IgG) antibodies may be passed to the fetus through the placenta. All neonates born to mothers who have reactive nontreponemal and treponemal test results should be evaluated with a quantitative nontreponemal serologic test (RPR or VDRL) on neonatal serum. The nontreponemal test that is performed in the infant should be the same as that which was performed in the mother so that the infant’s titers can be compared with the mother’s titers (treponemal testing on neonatal serum is not recommended because of the difficulty it presents for interpretation).  

An infant with a serum VDRL or RPR titer that is fourfold higher than the corresponding maternal titer is considered to have proven or highly probable congenital syphilis. Note that the absence of a fourfold or greater titer for the neonate does not exclude congenital syphilis, given that an abnormal physical examination that is consistent with congenital syphilis or a positive darkfield test/polymerase chain reaction (PCR) test of lesions or body fluids also aligns with the criteria for proven or highly probable congenital syphilis.  

Neurosyphilis

T. pallidum can infect the central nervous system and result in neurosyphilis. No single laboratory test can diagnose neurosyphilis. The diagnosis depends on a combination of CSF studies in the presence of reactive serologic test results and neurologic signs and symptoms. In a person with neurologic signs or symptoms, a reactive CSF VDRL is considered diagnostic. Although the FTA-ABS CSF test is less specific but more sensitive than VDRL for neurosyphilis, it is not generally recommended for diagnosing neurosyphilis. 

Screening

Nonpregnant Adults and Adolescents

The U.S. Preventive Services Task Force (USPSTF) recommends screening for syphilis infection in persons who are at increased risk for infection. Men who have sex with men or persons living with HIV may benefit from more frequent screening (every 3 months) but should be screened at least annually. 

HIV-Infected Individuals

Individuals infected with HIV should be screened at an initial visit and then annually, if sexually active. More frequent screening (every 3-6 months) is recommended for those with multiple sex partners and for those who engage in sex and/or illicit drug use.  

Pregnant Women

Most major national organizations, including the CDC, recommend screening all pregnant women for syphilis at the first prenatal visit. Additionally, the CDC, the American Academy of Pediatrics (AAP), and the American College of Obstetricians and Gynecologists (ACOG) endorse repeat screening for women at high risk for syphilis and recommend rescreening early in the third trimester and again at delivery.  

Monitoring Therapy Response

Titers from nontreponemal tests (RPR or VDRL) should be used to monitor treatment response. The frequency of monitoring depends on the stage of disease and the presence of HIV coinfection.  

ARUP Laboratory Tests

Refer to the Syphilis Testing algorithm for more information.

Preferred Testing for Screening and Diagnosis of Syphilis

Recommended test for syphilis screening and diagnosis (traditional algorithm)

Recommended confirmation test for patients with reactive nontreponemal screening test (traditional algorithm)

Recommended test for syphilis screening (reverse algorithm)

Recommended confirmation test for patients with reactive treponemal screening test (reverse algorithm)

Preferred Monitoring Test

Recommended test for monitoring treatment response in patients with established syphilis

Preferred Testing for Neurosyphilis

Preferred diagnostic assay for CSF specimens in cases of suspected neurosyphilis

Preferred Testing for Congenital Syphilis
Other Available Tests

Medical Experts

Author

Slev

Patricia R. Slev, PhD
Associate Professor of Clinical Pathology, University of Utah
Section Chief, Immunology; Medical Director, Immunology Core Laboratory, ARUP Laboratories
Medical Director, Serologic Hepatitis and Retrovirus and Immunology Core Laboratory
Co-Medical Director, Microbial Immunology, at ARUP Laboratories

References

Additional Resources