Syphilis Testing: Making Sense of Titers

Case

A 26-year-old patient is seen for follow-up. They were treated for secondary syphilis one year ago. Their initial RPR upon treatment was 1:32; 6 months later it was 1:8; today it is 1:64. What’s the significance of the titer changes over time?

Serology

There are 2 types of serologic tests: 

  • Nontreponemal tests detect antibodies directed against lipoidal antigens (cardiolipin, cholesterol, and lecithin; typically not detected until ~6 weeks after infection):
    • Rapid plasma reagin (RPR)
    • Venereal Disease Research Laboratory (VDRL)
    • Toluidine Red Unheated Serum Test (TRUST)
  • Treponemal tests detect antibodies directed against specific treponemal antigens (more specific; typically not detected until ~3 weeks after infection):
    • Fluorescent treponemal antibody absorption (FTA-ABS)
    • Microhemagglutination test for antibodies to Treponema pallidum (MHA-TP)
    • T. pallidum particle agglutination assay (TPPA)
    • T. pallidum enzyme immunoassay (TP-EIA)
    • Chemiluminescence immunoassay (CIA)
Source: Soreng et al, Clin Microbiol Newsl. 2014

Nontreponemal tests are quantitative (reported as titers).

  • A fourfold change in titer, equivalent to a change of two dilutions (e.g., from 1:16 to 1:4, or from 1:8 to 1:32), is considered a clinically significant difference between results.
  • Titers can potentially increase in the interval between diagnosis and treatment initiation, so it’s important to repeat testing upon treatment initiation.
  • For patients with a history of treated syphilis, the presence of a positive nontreponemal test can indicate a new infection, an evolving response to treatment, or treatment failure.
  • Titers tend to wane over time even without treatment, but successful treatment accelerates the pace of antibody decline (see figure below).

Treponemal tests are qualitative (reported as reactive or nonreactive).

Source: Soreng et al, Clin Microbiol Newsl. 2014

Diagnosis

Confirming a diagnosis of syphilis requires both a nontreponemal test and a treponemal test because false-positive testing can occur with either type (particularly the former.) As such, Lifespan’s syphilis screening algorithm follows CDC recommendations

  • For patients with no known history of syphilis, first check Treponema pallidum IgM/IgG; a positive or indeterminate/equivocal test result will reflex to an RPR test.
Source: Soreng et al, Clin Microbiol Newsl. 2014

Treatment Follow-up

Patients diagnosed with syphilis should have clinical and serologic evaluation repeated at 6 months and 12 months after treatment to assess treatment response and potential re-infection from subsequent exposure. 

Nontreponemal tests are valuable for measuring the efficacy of drug treatment because they are quantitative. A fourfold drop in titer is indicative of successful antibiotic treatment, and the nontreponemal antibody titer should be virtually undetectable following effective antibiotic treatment in most patients.

Serologic response to treatment appears to be associated with multiple factors:

  • Syphilis stage (earlier stages are more likely to decrease fourfold and become nonreactive)
  • Initial nontreponemal titers (<1:8 are less likely to decline fourfold than higher titers)
  • Age (older patients might be less likely to decrease fourfold)

“Serofast” patients fail to fully resolve serologically and perpetually exhibit low nontreponemal titers; re-infected pts have persistently higher titers.

Failure of titers to decrease fourfold within 12 months after treatment for primary or secondary syphilis might be indicative of treatment failure; however, 10-20% of pts with primary and secondary syphilis who received recommended treatment will not achieve the fourfold decrease in nontreponemal titer within 12 months after treatment.

There is no immunity associated with successful treatment, so reinfection can occur with subsequent exposure.

Other Considerations

There are MANY diseases and conditions that can cause false-positive results with nontreponemal and treponemal tests:

  • Nontreponemal tests:
    • Inflammation
    • Autoimmune diseases (especially systemic lupus erythematosus)
    • Acute viral infections
    • Hepatitis C
    • Pregnancy
    • Recent immunization
    • Connective tissue diseases
    • HIV infection
    • Injection drug use
    • Malignancy
    • Advancing age
  • Treponemal tests:
    • Thyroiditis
    • Systemic lupus erythematosus
    • Scleroderma
    • Infectious mononucleosis
    • Genital herpes
    • Cirrhosis
    • Pregnancy
    • Recent immunization
    • Hyperglobulinemia
    • Bacterial infections (e.g., brucellosis, leptospirosis, Lyme disease, malaria, Hansen’s disease)
    • Injection drug use
    • Yaws, pinta, bejel
    • Advancing age

False-negative results can also occur (rarely) with nontreponemal tests because of the prozone phenomenon (also known as the hook effect, antibody excess, or postzone phenomenon):

  • The prozone phenomenon, which is uncommon, occurs when the nontreponemal antibody concentration is very high. The test antigen becomes saturated by nontreponemal antibodies, preventing formation of the antigen-antibody matrix required for agglutination.
  • This effect can be avoided by serially diluting a sample until the antibody titer is low enough to yield a positive response. This approach should be performed if clinical suspicion of syphilis is high and the specimen yields a serologic result that is weakly reactive or atypical.

Case Conclusion

Our patient’s RPR titer decreased fourfold following initial treatment (from 1:32 to 1:8), consistent with a successful response to treatment. The subsequent increase in titer (from 1:8 to 1:64) likely suggests reinfection, and would require repeating treatment.