Syphilis Serology

Syphilis, caused by the spirochaete Treponema pallidum, cannot be cultured, so diagnosis depends on serology (and on direct detection from lesions in primary and secondary disease). Two types of antibody tests are used.

• Non-treponemal tests (rapid plasma reagin, RPR; Venereal Disease Research Laboratory, VDRL) detect antibodies to cardiolipin-lecithin-cholesterol antigen. They are reported as titres and track disease activity and treatment response.
• Treponemal tests (T. pallidum particle agglutination, TPPA; enzyme and chemiluminescence immunoassays, EIA and CIA; fluorescent treponemal antibody absorption, FTA-ABS; rapid point-of-care tests) detect antibodies specific to T. pallidum. They stay positive for life in most people and confirm exposure.

About 8 million new adult cases occur worldwide each year, and rates are rising in many countries. Untreated syphilis progresses from primary (chancre) to secondary (rash, condylomata lata, lymphadenopathy), latent and tertiary stages (cardiovascular, gummatous, neurological). Congenital syphilis causes over 300 000 adverse pregnancy outcomes yearly, including stillbirth and neonatal death.

Indications.

• Genital or oral ulcers.
• Rash on the trunk, palms and soles, or generalised lymphadenopathy.
• Neurological or ocular symptoms in a person with risk.
• Antenatal screening (first visit, and repeated at 28 to 32 weeks and at delivery in high risk areas).
• Sexually transmitted infection screening, HIV infection, partner notification and blood donation.
• Unexplained cranial nerve palsy, uveitis or hearing loss.
• Neonates of infected mothers.
• Dementia or aortitis in older adults.

Specimen. Serum (or plasma), 3 to 5 mL. For neurosyphilis, cerebrospinal fluid. For primary lesions, swab or exudate for dark field microscopy or PCR. Whole blood for rapid treponemal tests.

Testing algorithms.

• Traditional: non-treponemal screen (RPR/VDRL) then treponemal confirmation (TPPA).
• Reverse sequence: treponemal screen (EIA/CIA) then non-treponemal test. If the treponemal test is reactive and non-treponemal is negative, a second, different treponemal test (TPPA) resolves the discrepancy.
• Point-of-care treponemal rapid tests are used in antenatal and community settings, followed by RPR to direct treatment.

Interpretation.

Titres and response. RPR titre is reported as a dilution (1:1, 1:2, 1:4... 1:256). A fourfold (two dilution) change is significant. After adequate treatment, titres should fall at least fourfold by 6 to 12 months in early syphilis (primary, secondary) and 12 to 24 months in latent disease. A fourfold rise suggests reinfection or relapse. Many people remain serofast (low titre, 1:1 to 1:4) for life, and this is not treatment failure. Titres in secondary syphilis are often 1:32 or higher.

Direct detection. Dark field microscopy of chancre exudate shows motile spirochaetes (needs immediate examination and experience), and PCR or direct fluorescent antibody are alternatives. Primary syphilis with a negative serology in the first weeks is common and repeated after 2 to 4 weeks.

Neurosyphilis. Diagnosed by CSF: reactive CSF VDRL (specific, 30 to 70% sensitive), CSF white cells over 5 cells/microL, and raised protein. Lumbar puncture is done with neurological, ocular or otic symptoms, tertiary disease, treatment failure, or HIV with late latent syphilis and high RPR titres.

Biological false positives of non-treponemal tests. Pregnancy, autoimmune disease (systemic lupus erythematosus, antiphospholipid syndrome), HIV, viral infections (hepatitis, infectious mononucleosis, measles, varicella), vaccination, tuberculosis, malaria, leprosy, injecting drug use, older age, and malignancy. Titres are usually under 1:8, and a treponemal test is negative.

Congenital syphilis. Non-treponemal titre in the newborn fourfold higher than the mother's, or a rising infant titre, with clinical and radiological signs, supports infection. Maternal treponemal IgG crosses the placenta, so an infant treponemal test is not diagnostic before 15 to 18 months. IgM tests, PCR, and CSF testing are used.

Critical values and findings needing immediate action.

• Reactive treponemal and non-treponemal tests in pregnancy: penicillin treatment at once, ideally 30 days before delivery, and infant evaluation.
• Neurological, ocular or auditory symptoms with positive serology: lumbar puncture and intravenous penicillin.
• Syphilis in a patient with HIV: staging for neurosyphilis, and closer follow up.
• Fourfold titre rise after treatment: relapse or reinfection, retreatment, and CSF evaluation if neurological signs.
• Positive serology in a neonate with hepatosplenomegaly, rash, snuffles or bone changes: urgent paediatric care.
• Jarisch-Herxheimer reaction (fever, rigors within 24 hours of treatment) in early syphilis, and in pregnancy it may cause foetal distress.

Interferences and pitfalls.

• Prozone phenomenon: very high antibody concentrations in secondary syphilis give false negative non-treponemal results, corrected by dilution of the serum.
• Early primary syphilis: serology can be negative for up to 2 to 4 weeks, so a repeat test is done if the lesion is suspicious.
• Treponemal tests remain positive after cure, so they cannot detect reinfection. Non-treponemal titres are used.
• Different non-treponemal methods (RPR and VDRL) are not interchangeable, and the same test and laboratory are used for follow up.
• Other treponemal diseases (yaws, bejel, pinta) give positive treponemal and non-treponemal tests indistinguishable from syphilis.
• Lyme disease and leptospirosis can cause false positive treponemal tests.
• HIV may delay or alter serological responses, and rarely cause false negatives.
• An isolated reactive treponemal screen in a low risk person is often a false positive, and a second treponemal test should be done before diagnosis.
• Partners need testing and treatment.

Special groups.

• Pregnancy: universal screening, penicillin is the only reliable treatment to prevent congenital syphilis, and desensitisation is used in allergy.
• HIV coinfection: atypical presentations, faster progression to neurosyphilis, and slower titre decline.
• Men who have sex with men: high incidence and screening every 3 to 6 months.
• Older adults: tertiary syphilis (aortitis, tabes dorsalis, general paresis) is rare but presents with cognitive decline.
• Regions: endemic treponematoses in parts of Africa, Asia and the Pacific, and shortages of benzathine penicillin impede treatment.

Follow up tests.

• Repeat quantitative RPR at 3, 6, 12 and 24 months after treatment.
• HIV test, hepatitis B and C serology, gonorrhoea and chlamydia testing.
• Lumbar puncture with CSF VDRL, white cells and protein.
• Ophthalmological and audiological assessment in suspected ocular or otic syphilis.
• Chest radiograph and echocardiography in suspected cardiovascular syphilis.
• Infant long bone radiographs and ophthalmic exam in congenital syphilis.