A newborn born to a mother treated for early latent syphilis in the second trimester has a cord blood RPR titer of 1:32, identical to the mother's delivery titer. The infant is asymptomatic. Which finding would best confirm that the infant has congenital syphilis rather than passive transfer of maternal antibody?

• A Persistence of the same RPR titer at birth
• B A positive FTA-ABS in neonatal serum
• C Demonstration of treponemal IgM in neonatal serum ✓
• D Maternal TPPA positivity

Explanation

Maternal IgG crosses the placenta, so a positive neonatal RPR, FTA-ABS, or TPPA may reflect passively transferred maternal antibody rather than infection. Treponemal IgM does not cross the intact placenta because of its large pentameric structure, so demonstration of specific IgM (by IgM immunoblot or 19S-IgM-FTA-ABS) indicates fetal antibody production and true congenital infection. Serial RPR titers are also used, with a rising or persistently elevated titer beyond 3 months supporting infection.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.