Interpreting Syphilis Serology

False positive test result

A false positive is when the test results come back as positive, but the patient is negative for the infection.

False positive test results can occur with:

• Both treponemal and RPR tests.
• False positive treponemal tests may be identified in the laboratory as an unclear or indeterminate test result but confirmatory tests are usually conducted with another treponemal test before the result is reported as reactive or non-reactive.

The RPR test is read manually and therefore results may vary slightly among specimens taken at different times or processed at different laboratories. Biological false positive results can also occur with the RPR for a number of reasons including acute infections, inflammation, injecting drug use and pregnancy. False positive RPR results should only cause a low titre reading (usually <1:8) while laboratory errors or variations are generally only within 1 titre of each other. Reactive RPR results should always be considered as true positives among people at risk and an increase in RPR of two titres from the previous RPR considered a likely new infection.

False negative test result

A false negative is when the test results come back as negative, but the patient is actually positive for the infection.

False negative test results can occur among people who have:

• Very early syphilis and are tested before the test is measurable in the blood
• A prozone reaction where a large antibody (immune) response can make the test falsely negative
• Immune-suppression (e.g. due to HIV, chemotherapy, radiation, or immunosuppressant drugs, etc.

If there are any concerns about possible false positive results among people at low risk of infection, or false negative results among people likely to have syphilis always contact the regional PHU or appropriate specialists for advice and repeat testing.