What are the expected syphilis serology results, specifically Enzyme Immunoassay (EIA) and Rapid Plasma Reagin (RPR) titer, for a patient who has completed successful treatment for early-latent syphilis and has not had any subsequent sexual encounters?

Expected Syphilis Serology After Successful Treatment of Early-Latent Syphilis

The most reasonable expectation is EIA positive with RPR 1:4 or lower (Answer D), as treponemal tests remain positive for life while nontreponemal titers should decline at least fourfold after successful treatment. 1

Understanding Post-Treatment Serologic Patterns

Treponemal Test (EIA) Response

• Treponemal tests remain reactive for life in most patients regardless of treatment or disease activity 1, 2
• Only 15-25% of patients treated during primary syphilis may revert to serologically nonreactive after 2-3 years 1
• Since this patient had early-latent (not primary) syphilis, the likelihood of EIA reverting to negative is extremely low 1
• The EIA should remain positive in this patient 1, 2

Nontreponemal Test (RPR) Response

• After successful treatment, nontreponemal titers should decrease at least fourfold within 6-12 months for early syphilis 1
• The patient's initial RPR was 1:128, so a fourfold decline would be to 1:32, and an additional fourfold decline would be to 1:8 3, 1
• After 18 months post-treatment, most patients will have achieved at least a fourfold decline, often more 1, 4
• Many patients become "serofast" with persistent low-level titers (generally <1:8) for extended periods 3, 1

Clinical Context Supporting Answer D

Expected Titer Decline Timeline

• By 6-12 months: At least fourfold decline expected (from 1:128 to ≤1:32) 1
• By 18 months: Further decline anticipated, with many patients reaching low titers (1:4 to 1:8) or seroreversion 1, 4
• An RPR of 1:4 at 18 months represents appropriate serologic response to treatment 3, 1

Why Other Options Are Incorrect

Option A (EIA negative, RPR non-reactive):

• Treponemal tests rarely revert to negative, especially in latent syphilis 1, 2
• Complete RPR seroreversion occurs in only 72% of primary syphilis cases by 36 months, and less frequently in latent disease 4

Option B (EIA negative, RPR 1:64):

• EIA should remain positive 1, 2
• RPR 1:64 represents only a twofold decline from baseline (1:128), which is inadequate response 1

Option C (EIA positive, RPR 1:128):

• While EIA positivity is correct, unchanged RPR titer at 18 months indicates treatment failure 3, 1
• Lack of fourfold decline by 6-12 months warrants retreatment consideration 3

Important Clinical Considerations

Serofast State

• Approximately 15-20% of successfully treated patients remain serofast with persistent low titers (<1:8) 3, 1
• Serofast state does not represent treatment failure 3, 1
• Reinfection is diagnosed by at least fourfold increase above the serofast baseline 3, 1

HIV-Specific Factors

• HIV-infected patients may have slower or atypical serologic responses 3, 5, 6
• Patients with baseline RPR ≤1:16 are at higher risk for serologic failure 5
• However, this patient had high initial titer (1:128), making adequate response more likely 5
• The patient should still achieve at least fourfold decline despite HIV infection 3, 6

Monitoring Pitfalls to Avoid

• Do not compare titers between different test methods (VDRL vs RPR) 1, 2
• Do not assume persistent low-titer reactivity indicates treatment failure 3, 1
• A fourfold change in titer (two dilutions) is clinically significant 1, 2
• Sequential tests should use the same method, preferably the same laboratory 1, 2