False Positives in Syphilis Testing: A Case Study

Is it possible for a treponemal test to yield a false positive result?

I am 36 years old.
In August, I donated blood and received a notification that my TPHA was positive and RPR was negative.
In January, my blood donation results were still normal, all negative.
In November, I went to the infectious disease department for retesting and received the same results: RPR negative (non-detectable value), and chemical cold light CIA positive (value 10.19).
I found online that there are some possible false positives for TPHA, but my doctor told me that these are extremely rare cases.
I find it unlikely that I fall into this category; I should have had an infection at some point.
However, it is strange that between January and August, I had no symptoms and no sexual activity.
Even if I had an infection unknowingly and it resolved on its own, it seems unlikely that both RPR tests would be completely negative; there should at least be some detectable titer.
Am I really one in a million with a unique constitution? Additionally, is it really impossible that I have leptospirosis? I have heard that some people may not show symptoms or only have mild cold-like symptoms.
I occasionally come into contact with stray dogs; is it really not possible? Furthermore, I am currently on a ketogenic diet and taking weight loss medication, which may temporarily lead to hepatitis or fatty liver; could this affect my test results? Additionally, I have a history of atopic dermatitis and my body fat percentage exceeds 35%.
I was stung by a bee in May and received a tetanus shot, and in July, I was diagnosed with COVID-19 and took antiviral medication.
I received the Moderna vaccine in January.

Jie Yu, 30~39 year old female. Ask Date: 2022/11/09

Dr. Hong Yuanbin reply Internal Medicine

Hello, the two indices for syphilis, RPR and TPHA, show that RPR is more sensitive and can easily yield false positives, which may turn negative after syphilis treatment.
On the other hand, the chance of false positives with TPHA is very low, and it typically remains positive even after successful treatment.
Regarding the leptospira you mentioned, factors such as diet, bees, and vaccines should not cause false positives.
Thank you for your question.

Reply Date: 2022/11/09

More Info

Understanding false positives in syphilis testing can be quite complex, especially when considering the nuances of individual health circumstances and the intricacies of serological testing. In your case, the results from your syphilis tests have raised several questions, particularly regarding the positive TPHA (Treponema pallidum hemagglutination assay) and negative RPR (Rapid Plasma Reagin) tests.

Firstly, it's essential to understand that the TPHA test is a more specific test for syphilis, while the RPR is a non-treponemal test that is often used for screening. A positive TPHA indicates that there are antibodies present against Treponema pallidum, the bacterium that causes syphilis. However, a positive TPHA does not necessarily confirm an active infection; it can indicate a past infection or even a false positive due to cross-reactivity with other conditions.

The negative RPR test is significant because it suggests that there is no active syphilis infection at the time of testing. This discrepancy between the TPHA and RPR results can occur for several reasons. One possibility is that you may have had a past syphilis infection that was treated or resolved on its own, which can lead to a positive TPHA while the RPR remains negative. This is particularly plausible given that you reported no symptoms or sexual activity during the period in question.

Regarding your concerns about leptospirosis (caused by Leptospira bacteria) from contact with stray dogs, while it is true that leptospirosis can sometimes present with mild or nonspecific symptoms, it typically does not cause false positives in syphilis testing. The serological tests for syphilis are designed to detect specific antibodies related to Treponema pallidum, and while there can be cross-reactivity with other infections, leptospirosis is not commonly associated with false positives in syphilis tests.

Your current health conditions, including the ketogenic diet and potential liver issues, are also worth considering. While liver function can affect various laboratory tests, it is unlikely to directly influence the results of syphilis serology tests. However, if you are experiencing liver dysfunction, it is essential to monitor your overall health and consult with a healthcare provider regarding any medications or dietary changes.

In summary, the combination of a positive TPHA and negative RPR suggests that you may have had a past syphilis infection, but it does not indicate an active infection at this time. It is advisable to follow up with your healthcare provider for further evaluation and possibly repeat testing in the future to monitor your antibody levels. Additionally, if you have concerns about other infections or health issues, discussing these with your doctor can provide clarity and ensure appropriate management. Regular screenings and open communication with your healthcare provider are key to maintaining your sexual health and overall well-being.

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