Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum. If left untreated, it can cause serious health problems. Diagnosing syphilis relies heavily on blood tests. But how accurate are these blood tests for detecting syphilis?
Types of syphilis blood tests
There are two main types of blood tests for syphilis:
- Nontreponemal tests: These tests check for antibodies produced by the body in response to a syphilis infection. Examples include the Venereal Disease Research Laboratory (VDRL) test and the rapid plasma reagin (RPR) test.
- Treponemal tests: These tests check for antibodies produced in response to the bacterium that causes syphilis. Examples include the fluorescent treponemal antibody absorption (FTA-ABS) test, the T. pallidum passive particle agglutination (TP-PA) test, and treponemal enzyme immunoassays (EIAs).
Treponemal tests are more specific for syphilis than nontreponemal tests. Often, a nontreponemal screening test is done first, followed by a treponemal confirmatory test if the screening test is positive.
Accuracy of syphilis blood tests
The accuracy of syphilis blood tests is usually measured by their sensitivity and specificity:
- Sensitivity refers to the ability of a test to correctly identify people who have syphilis (true positives).
- Specificity refers to the ability of a test to correctly identify people who do not have syphilis (true negatives).
In general, treponemal tests have a sensitivity and specificity of >90% in all stages of syphilis. This means they are highly accurate at both detecting syphilis and avoiding false positives.
Nontreponemal tests are less accurate. The sensitivity of these tests depends on the stage of syphilis:
- Primary syphilis: Sensitivity 30-70%
- Secondary syphilis: Sensitivity 75-100%
- Latent syphilis: Sensitivity ~100%
- Tertiary syphilis: Sensitivity 71-100%
So nontreponemal tests can miss 30-70% of primary syphilis cases. However, they have a high specificity of ~98%. False positives are uncommon but can occur.
Factors affecting test accuracy
Several factors can affect the accuracy of syphilis blood tests:
- Proper specimen collection and handling – Incorrect collection or handling of blood can reduce accuracy.
- Stage of infection – Tests are least accurate in early primary syphilis.
- Past syphilis infection – Old antibodies may cause false positive nontreponemal tests.
- Other conditions – Some other diseases can cause false positive nontreponemal tests.
To maximize accuracy, both a treponemal and nontreponemal test should be done according to proper protocols.
Interpreting syphilis test results
Interpreting syphilis blood test results can be complex. Here are some key points:
- A reactive (positive) treponemal test confirms syphilis infection now or in the past.
- A negative treponemal test rules out syphilis unless there is strong clinical suspicion of very early primary syphilis.
- A reactive nontreponemal test suggests active syphilis infection but requires confirmation with a treponemal test.
- A negative nontreponemal test does not rule out syphilis entirely.
Often both types of tests are used together. The table below summarizes interpretation of possible results:
| Nontreponemal Test | Treponemal Test | Interpretation |
|---|---|---|
| Nonreactive | Nonreactive | Syphilis highly unlikely |
| Reactive | Nonreactive | False positive screening test |
| Nonreactive | Reactive | Past treated syphilis or very early primary syphilis possible |
| Reactive | Reactive | Active syphilis infection likely |
If syphilis remains suspected, repeat testing in 1-2 weeks may be needed to confirm or exclude the diagnosis.
Conclusion
Treponemal blood tests for syphilis antibodies are highly accurate for both ruling in and ruling out syphilis infection. However, false positives and false negatives can occasionally occur. Nontreponemal tests are less sensitive, especially in early syphilis. To maximize diagnostic accuracy, most experts recommend using both a nontreponemal screening test followed by a treponemal confirmatory test when investigating possible syphilis. Proper test procedures and close correlation with syphilis signs/symptoms are also key for optimal accuracy.