A 75 year old man presents to hospital with a cerebro-vascular accident.
As part of his routine laboratory investigations syphilis serology is performed.
The results show a positive TPPA titre of 1:128. RPR testing is negative.
Question 1: Could this TPPA result be falsely positive?
It could be, but very unlikely. Specific treponemal antibody tests such as TPPA tend not to give false positive results. On the rare occasion that they do, it is usually due to laboratory error or an autoimmune condition In contrast non-specific treponemal antibody tests such as RPR or VDRL are quite prone to giving low titre false positive results. False positive RPR or VDRL results can be caused by variety of acute and chronic diseases, such as mixed connective tissue disease, autoimmune disease, diabetes mellitus, alcoholic cirrhosis, viral infections and pregnancy.
Question 2: What clinical scenarios could give rise to this serological picture?
Quite a few…
- Probably the most common reason for this picture is past treated syphilis. However late latent untreated infection can also give this picture, as in such cases the VDRL/RPR antibody can fall away to undetectable limits.
- In persons from countries where other treponemal infections such as yaws (eg Pacific, Carribean), pinta (Central America) or bejel are endemic, then infection with these conditions can give positive treponemal antibody results.
- The other possible scenario is early primary syphilis infection. One would have to say that this unlikley, but not impossible in a 75 year old!