Syphilis (Part 2)

Written by: Solaris Diagnostics

Laboratory Diagnosis of Syphilis

Two antibody types are detected to determine syphilis diagnosis:

A. Treponemal antibodies 

Antibodies directed against T. pallidum. 

Treponemal serologic tests includes:

  • Treponema pallidum particle agglutination (TP-PA) 
  • Enzyme immunoassay (EIA) 
  • Chemiluminescent immunoassay (CLIA)
  • Fluorescent treponemal antibody absorption (FTA-ABS)  

B. Non-treponemal antibodies

Antibodies directed against antigens such as cardiolipin, cholesterol, and lecithin – Released in response to T. pallidum active infection

Non-treponemal serologic tests: Usually a quantitative test to evaluate treatment progress.

  • Venereal Disease Research Laboratory (VDRL)
  • Rapid plasma regain (RPR)
  • Unheated serum regain (USR) 
  • Toluidine red unheated serum (TRUST)

Nontreponemal Tests and Traditional Algorithm

False-positive nontreponemal test results can be associated with multiple medical conditions and factors unrelated to syphilis, including other infections (e.g., HIV), autoimmune conditions, vaccinations, injecting drug use, pregnancy, and older age. Therefore, persons with a reactive nontreponemal test should always receive a treponemal test to confirm the syphilis diagnosis (i.e., traditional algorithm). 

Nontreponemal test antibody titers might correlate with disease activity and are used for monitoring treatment response. Serum should be diluted to identify the highest titer, and results should be reported quantitatively. 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 necessary for demonstrating a clinically significant difference between two nontreponemal test results obtained by using the same serologic test, preferably from the same manufacturer to avoid variation in results.

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