Syphilis Serology Interpretation
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Direct answer
One rule anchors syphilis serology: non-treponemal tests screen and monitor, treponemal tests confirm. VDRL and RPR use cardiolipin antigens, give quantitative titres that should fall four-fold after adequate treatment, and are prone to biological false positives in malaria, leprosy, pregnancy and HIV; TPHA, FTA-ABS and treponemal immunoassays confirm specificity but usually stay positive for life. A negative VDRL with lesions suggesting secondary syphilis should trigger suspicion of the prozone effect — dilute and retest — and any discussion of treatment must include the Jarisch-Herxheimer reaction, not an allergy, for NEET-PG Medicine purposes.
What you must remember
- Non-treponemal (VDRL, RPR): flocculation tests with cardiolipin–cholesterol–lecithin antigen; quantitative, cheap, ideal for screening and post-treatment monitoring.
- Treponemal (TPHA/TPPA, FTA-ABS, EIA/CLIA): specific antibodies that typically persist for life; confirmatory, not for monitoring.
- Titre response: four-fold (two-dilution) decline expected by 6–12 months in early syphilis after correct therapy; failure demands re-evaluation for reinfection or neurosyphilis.
- Biological false-positive VDRL: transient (<6 months) after recent vaccination, malaria, infectious mononucleosis; chronic with leprosy, autoimmune disease, injecting drug use — in India, leprosy and malaria are the classical exam answers.
- Prozone phenomenon: excess antibody in secondary syphilis, HIV and pregnancy blocks flocculation, producing a false-negative VDRL; diluting the serum restores positivity.
- Reverse-sequence screening (treponemal immunoassay first) needs a reflex RPR and a confirmatory TPHA for discordant results.
- CSF-VDRL is highly specific but insensitive; CSF pleocytosis and protein support neurosyphilis when CSF-VDRL is negative.
- Jarisch-Herxheimer: fever, chills, myalgia, tachycardia and hypotension within 2–8 hours of first-dose penicillin, commonest in early syphilis, self-limiting over 12–24 hours; antihistamines do not prevent it.
- Caution scenarios for Herxheimer: neurosyphilis, otosyphilis, pregnancy (fetal and maternal monitoring) and cardiovascular syphilis.
- Treatment of early syphilis: single dose benzathine penicillin G 2.4 million units intramuscularly; follow-up serology at 6 and 12 months.
How to work through a typical result
A 28-year-old pregnant woman at 24 weeks has a reactive VDRL at 1:16; TPHA is positive; she recalls a painless genital ulcer two months ago that healed untreated.
Step one: interpret the pair — reactive VDRL with confirmatory TPHA in a patient with a recent chancre means untreated early latent or secondary syphilis; quantify and stage (examine for rash, condylomata lata, mucous patches, lymphadenopathy). Step two: treat without delay — benzathine penicillin G 2.4 million units intramuscularly is the drug of choice in pregnancy, and desensitisation is the pathway for true penicillin allergy, not substitution. Step three: warn and prepare for the Jarisch-Herxheimer reaction — fever and uterine contractions can occur within hours; arrange fetal monitoring during the first dose in the third trimester, and reassure that it is self-limiting and that antihistamines do not abort it. Step four: monitor the titres — a four-fold fall by six to twelve months defines cure; in pregnancy, repeat monthly titres until delivery per programme guidance, and treat the neonate if maternal titres are rising at delivery or inadequate treatment was given. Step five: flip the scenario for the exam — were her lesions florid with a negative VDRL, the prozone effect would lead you to dilute the serum; were her VDRL positive with negative TPHA and a stable low titre, a biological false positive from coexistent leprosy or malaria would head the differential.
Where students slip
First, using a falling TPHA to judge cure — treponemal tests persist for years, and only quantitative VDRL/RPR titres monitor response. Second, ignoring the prozone: a stem of "florid secondary syphilis, VDRL negative" is practically a coded instruction to dilute the serum. Third, mistaking Jarisch-Herxheimer for penicillin allergy and switching drugs; the expected answer is supportive care, not desensitisation. Fourth, forgetting that CSF-VDRL's insensitivity means a negative result with pleocytosis and protein can still mean neurosyphilis. Finally, candidates quote follow-up at arbitrary intervals; six and twelve months for early syphilis is the standard answer.
Frequently asked questions
Which syphilis test is used to monitor treatment response?
A quantitative non-treponemal test — VDRL or RPR — with a four-fold (two-dilution) titre decline expected within 6–12 months of adequate therapy for early syphilis.
What is the prozone phenomenon in VDRL?
Excess antibody in secondary syphilis, HIV or pregnancy prevents flocculation, giving a false-negative result that reverses on diluting the serum before retesting.
Which conditions cause biological false-positive VDRL results?
Transiently after vaccination, malaria and infectious mononucleosis; chronically with leprosy, autoimmune disease and injecting drug use — leprosy and malaria being classical Indian associations.
What is the Jarisch-Herxheimer reaction?
A self-limiting cytokine reaction occurring within 2–8 hours of the first antibiotic dose in early syphilis — fever, chills, myalgia and hypotension — managed supportively, with fetal monitoring when it occurs in pregnancy.
How is discordant reverse-sequence screening resolved?
A reactive treponemal immunoassay with a negative RPR is confirmed by TPHA/TPPA; a positive confirmatory result with a negative RPR suggests previously treated, late or latent infection and is managed by staging.