Syphilis — Blood Tests

Syphilis is caused by the bacteria Treponema pallidum, which is impossible to grow in culture.ย  It also can’t be seen without a special “darkfield” microscope, which only a very sophisticated STD clinic might possess.ย  We virtually always have to rely on blood tests for antibodies.

The following discussion is fairly technical; itโ€™s included because many clinicians donโ€™t quite understand it.

There are 2 kinds of antibody tests for Syphilis:

  • “Non-Treponemal” —  RPR or VDRL
  • “Treponemal” —  TP-PA, MHA-TP, FTA-ABS, and a new test EIA.

Usually only the Non-Treponemal test is ordered.ย  The following are basic principles in understanding the tests

  • Treponemal tests are reported positive or negative, & stay positive forever.
  • Non-Treponemal are reported by strength (titers): 1:2 is low, 1:32 is high (read as “1-to-32,” etc.).ย 
  • With treatment, Non-Treponemal titers keep going down, may turn negative or may remain at a low number.ย  They can also become normal without treatment, after many years (so if we’re worried about Tertiary [end-stage] Syphilis, we need to order both kinds).
  • If reinfected later on, Non-Treponemal titers rise again.
  • Non-Treponemal tests may be false-positive; we know this if the Treponemal test is negative (labs routinely do Treponemal tests when the Non-Treponemal is positive).ย  Causes of false-positive RPR or VDRL include pregnancy, various acute infections, Lupus and other such diseases, HIV and other chronic infections.
  • All blood tests are negative during Primary Syphilis.ย  Diagnosis can only be made by darkfield microscope, which is rarely done.ย  Antibodies begin to turn positive 2-4 months after getting infected.ย 

We use blood tests to diagnose syphilis depending on the stage we think a person may have:

  • Primary Syphilis: a painless chancre sore (not painful “canker sore”) on genitals, anus, or mouthย  —ย  blood tests rarely help until weeks have gone by
  • Secondary Syphilis (rash, fevers, swollen glands, etc.)ย  —ย  Both Non-Treponemal & Treponemal tests are always positive.
  • No symptoms, just want to see if may have ever had syphilis that’s never been treatedย  —ย  a Treponemal test is best, though Non-Treponemal tests are OK & commonly ordered.
  • Tertiary Syphilis:ย  Both Non-Treponemal & Treponemal tests should be ordered.ย  The most common test for syphilis, the non-treponemal, are false-negative 25% of the time in Tertiary disease, so the Non-Treponemal has to be ordered specifically.ย  Most clinicians donโ€™t know this.

Actually, most clinicians don’t realize most of the above.ย  See also our topic, Syphilis.

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