Syphilis is caused by the bacteria Treponema pallidum, which is impossible to grow in culture. It also can’t be seen without a special “darkfield” microscope, which only a very sophisticated STD clinic might possess. We virtually always have to rely on blood tests for antibodies.
The following discussion is fairly technical; it’s included because many clinicians don’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. 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.).
- With treatment, Non-Treponemal titers keep going down, may turn negative or may remain at a low number. 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). 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. Diagnosis can only be made by darkfield microscope, which is rarely done. Antibodies begin to turn positive 2-4 months after getting infected.
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 mouth — blood tests rarely help until weeks have gone by
- Secondary Syphilis (rash, fevers, swollen glands, etc.) — Both Non-Treponemal & Treponemal tests are always positive.
- No symptoms, just want to see if may have ever had syphilis that’s never been treated — a Treponemal test is best, though Non-Treponemal tests are OK & commonly ordered.
- Tertiary Syphilis: Both Non-Treponemal & Treponemal tests should be ordered. 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. Most clinicians don’t know this.
Actually, most clinicians don’t realize most of the above. See also our topic, Syphilis.