Treponema pallidum - Syphilis | Choose the Right Test

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Author

Slev

Professor of Pathology (Clinical), University of Utah

Section Chief, Immunology; Medical Director, Immunology Core Laboratory, ARUP Laboratories

The Treponema pallidum subspecies pallidum is the etiologic agent of syphilis. Syphilis is usually transmitted sexually, but it 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 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. It's recommended that one of two testing algorithms be used: the traditional algorithm 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 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.

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 individuals, 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. Use of one of two testing algorithms is currently recommended: 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 tests can be qualitative or quantitative. The VDRL test can be performed on serum or cerebrospinal fluid (CSF). VDRL tests performed on CSF are 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 the 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 This approach often misses early primary or treated infection Moderately high rate of false-positive results for the initial nontreponemal test (reactive test requires confirmation with treponemal test)	Treponemal antibody reactivity persists over a lifetime This 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 a second treponemal test
Testing methodologies	RPR, VDRL	EIA, CIA, TP-PA, FTA-ABS
PPV, positive predictive value Sources: Workowski, 2021; 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 test 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 or anonymous sex partners.^,

Pregnant Individuals

Most major national organizations, including the CDC, recommend screening all pregnant individuals 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 individuals 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.^,