Reactive Syphilis Screen with a Nonreactive RPR in Pregnancy
A 29-year-old woman at 10 weeks' gestation has routine prenatal screening. Her syphilis screening EIA (IgG/IgM) is reactive with an index of 3.4. Reflex RPR is nonreactive on undiluted and 1:1 serum. Reflex TP-PA is reactive at 4+. She reports no history of syphilis, no genital lesions, no rash, and no prior treatment that she knows of; her only medication is a prenatal vitamin. She is married, reports a single partner, and her HIV test is negative. She asks whether she has syphilis and whether her baby is at risk.
1. Interpret the three results. What does this pattern establish?
Read the worked answer
This is the reverse screening algorithm: a treponemal EIA is the first test, and a reactive result is followed by a nontreponemal test (RPR), with a second treponemal test (TP-PA) used to arbitrate.
What the pattern shows:
- Two different treponemal assays are reactive, so this is not a single false-positive EIA. Treponemal antibodies are present and typically remain reactive for life after any treponemal infection.
- The RPR is nonreactive, so there is no measurable nontreponemal antibody activity and no titer to follow.
What it does not establish:
- It cannot distinguish a remote, adequately treated infection from late latent syphilis or syphilis of unknown duration.
- A nonreactive RPR does not exclude early primary syphilis (the nontreponemal test may lag by up to 2 weeks), but the absence of lesions and the reactive TP-PA make this unlikely.
- Prozone is effectively excluded because the RPR was also tested at 1:1, though a high-titer prozone usually shows reactivity at higher dilutions rather than none.
A false-positive treponemal result remains possible (lupus, antiphospholipid syndrome, other spirochetal exposure) but two reactive treponemal assays make it unlikely.
2. What additional information and testing do you need before deciding on treatment?
Read the worked answer
The decision turns on whether a documented, adequately treated prior infection exists. Two reactive treponemal tests without a treatment record cannot be assumed to be old treated disease.
History to obtain:
- Any prior reactive syphilis test, and any documented course of benzathine penicillin, including doses and dates.
- Previous pregnancies and any antenatal records, including prior RPR titers.
- Partner history and any symptoms in the partner.
- Prior treatment for other STIs and any history of genital ulceration or rash.
Examination and testing:
- Full skin and genital examination for primary or secondary lesions.
- HIV testing (already negative) and offer screening for other STIs.
- A fresh RPR to confirm the nonreactive result, with dilutions reported.
- Baseline liver function tests and a documented nontreponemal titer of record.
Interpretation: without a documented treatment history, this must be managed as latent syphilis of unknown duration, which in pregnancy is treated as late latent.
3. How would you treat her, and what follow-up would you arrange?
Read the worked answer
Treatment: benzathine penicillin G 2.4 million units intramuscularly, once weekly for three doses, for late latent or unknown-duration syphilis. Pregnancy does not change the regimen, but adherence to the full three-dose course is critical - missed doses require restarting the series.
Before treatment: document informed consent and explain that treatment is being given for presumed latent infection, not because an active lesion was found.
Follow-up:
- Quantitative RPR at 28-32 weeks and again at delivery, because reinfection or treatment failure must be detected; in pregnancy, titers may be checked monthly after treatment.
- A fourfold fall in titer is expected after treatment; failure to fall or a rising titer requires re-evaluation, retreatment, and assessment of the partner.
- Partner notification and treatment of the partner.
- Neonatal evaluation and coordination with paediatrics at delivery.
If she reports a penicillin allergy: do not substitute doxycycline, which is contraindicated in pregnancy. Refer for penicillin desensitisation and treat with benzathine penicillin.