The result of the syphilis serology came back showing:
EIA positive not confirmed in second assay
EIA and RPR likely to be false positives.
Treponemal infection unlikely but please repeat to confirm.
The GP stared at the result for a few minutes before rolling his eyes to the ceiling and saying “what does this all mean; why the heck did I do this test?” Good questions!
For me there are a number of problems with the interpretation of syphilis serology results:
- All of the serology results can have false positives and false negatives in almost any combination
- Syphilis serology results can remain positive for many years after acute infection with or without treatment
- Syphilis serology tests take 2-4 weeks to become positive in acute infection therefore if initial tests are negative and acute syphilis is still suspected then the tests should be repeated 2-4 weeks later.
- Syphilis can mimic almost any disease and so it can be almost impossible to interpret the results “in the clinical context” as theoretically anything could be syphilis
- What are referred to as ”syphilis serology results” are actually tests for a group of related treponemal infections (a genus of spiral-shaped bacteria) including yaws, bejel, pinta and syphilis (Treponema pallidum), all of which can give positive results but have different clinical implications
What types of “syphilis serology” tests are there?
The serological tests for syphilis are divided into treponemal and non-treponemal.
Treponemal tests detect antigens or antibodies against specific treponemal antigens and are qualitative, being either positive or negative. Examples of treponemal tests include T. pallidum enzyme immunoassay (EIA), T. pallidum particle agglutination assay (TPPA), T. pallidum haemagglutination assay (TPHA), and T. pallidum IgG or IgM immunoblot. The EIA and TPPA are the most commonly used. The disadvantage to treponemal tests is that the results remain positive after infection even with successful treatment.
Non-treponemal tests are not specific for treponemal infections; they detect antibodies that react to a cardiolipin-cholesterol-lecithin antigen in the serum from patients with these infections. There is a 1-2% false positive rate in these tests. The old version of this type of test was the VDRL (Venereal Disease Research Laboratory) test but the more modern and reliable equivalent test is the RPR (Rapid Plasma Reagin) test. The advantage of non-treponemal tests is that they are quantitative and can therefore be used to monitor response to treatment as the tests become negative with successful therapy. A drawback to non-treponemal tests is that the level decreases with time; 50% of patients with latent syphilis can actually have a negative RPR 30-40 years after the initial infection, even though they have syphilis.
Interpretation of results
Most UK laboratories start by screening the serum sample using a T. pallidum EIA. If the EIA is negative most labs go no further and report a negative test as the negative predictive value is about 98%. The report states “No evidence of treponemal infection, if high risk of recent infection within the past 4 weeks then repeat in 2-4 weeks”.
If the EIA is positive then this result needs confirming with a second treponemal test, usually TPPA, as well as an RPR. Although the RPR is used for monitoring treatment, it is done at this time before a positive diagnosis to speed up turnaround time; it could technically be done after getting a positive TPPA but this would take an extra day. If the TPPA is negative then a 2nd EIA is done, which will be different (e.g. a different manufacturer or a different part of the antigen or antibody being detected, see diagram below), or an IgG immunoblot can be used. The TPPA and the 2nd EIA test are done due to the high false positive rate of treponemal tests; a single treponemal test has a positive predictive value of only 10%. Using 2 or 3 treponemal tests helps increase the likelihood that a positive test is a true positive (each test is unreliable on its own but the more positives, the higher the probability of that infection).
If a sample is positive by EIA then an RPR (Rapid Plasma Reagin) test is done. This is measured as a dilution of the serum sample, reported as negative or positive neat, 2, 4, 8, 16, 32 or greater than 32, and is the number of times it is diluted and still remains positive (neat is the “weakest” positive and >32 is the “strongest” positive). This test is not specific to syphilis; however it can be used to monitor response to treatment in a patient who has syphilis as it becomes weaker with treatment.
If the TPPA and the RPR are negative the result is confirmed negative.
If the RPR is positive a 2nd EIA (3rd treponemal test) is performed irrespective of the TPPA result.
There are other treponemal infections that also give positive results in “syphilis” serology, which are endemic in the tropics especially in South America and Africa. It is impossible to tell the difference between these types of treponemal infections based on serology. Ultimately a full clinical evaluation of the patient including clinical manifestations, ethnic origin, travel history and risk factors will help decide whether a patient needs treatment or not.
How is syphilis treated?
The treatment of syphilis can be tricky and in my opinion should be discussed with experts in the management of sexually transmitted infections in Genitourinary Medicine. The British Association for Sexual Health and HIV (BASHH) produce excellent guidelines for all types of sexually transmitted infections including syphilis.
Should patients with memory loss or dementia be screened for syphilis?
Although it used to be taught to look for syphilis in dementia patients, the NICE guideline for investigating patients with possible dementia specifically say NOT to test for syphilis unless there is a history suggestive of possible exposure. This is probably because interpreting the results is so difficult and leads to a lot of confusion and distress! I would agree with this whole-heartedly and extend this advice to include patients with mild cognitive impairment as well. As the incidence of syphilis in the general population of the UK is low, unless there are specific concerns relating to past sexual exposure a positive result is likely to fall in to the category of “false positives” and cause more problems than it solves.
So having asked for the test on his patient the GP phoned the local Microbiologist for further advice. The unreliability of syphilis testing and complexity of result interpretation was explained, a result of EIA positive not confirmed in second assay, TPPA negative, RPR positive (e.g. Sample B above) reassured the GP that his patient almost certainly didn’t have syphilis but that if the GP and the patient wanted to be absolutely certain then a repeat sample could be tested. The repeated sample gave exactly the same result and therefore syphilis was excluded from the differential diagnosis for why the patient had developed memory loss.