Characterization of the classical biological false-positive reaction in the serological test for syphilis in the modern era

• Among the 63,765 tested blood samples, 206 (0.32%) had the CBFP reaction.
• An increased likelihood of the CBFP reaction was associated with female subjects.
• CBFP reaction in the RPR test was associated with subjects ≥ 80 years old.
• CBFP reaction in the RPR test was associated with subjects between 16 and 35 years old.
• CBFP was found in other diseases that had not been previously reported.

To characterize the CBFP reaction in the modern era, we analyzed the results of parallel rapid plasma reagin (RPR) and Treponema pallidum particle agglutination (TPPA) tests from a total of 63,765 blood samples obtained at Zhongshan Hospital in the Medical College of Xiamen University from May 2008 to February 2013. Among the 63,765 tested blood samples, 206 (0.32%) had the CBFP reaction. In multivariate analysis, an increased likelihood of the CBFP reaction was associated with female subjects, subjects ≥ 80 years old, and subjects between 16 and 35 years old (P < 0.05). The CBFP reaction occurred in association with 17 categories of disease, including 60 types of diseases, in the 206 subjects. To our knowledge, a number of these diseases had not been previously reported to be associated with the CBFP in the RPR test, including false labor, megaloblastic anemias, aplastic anemias, redundant prepuce, congenital malformation of heart, and salpingitis. Among the 206 patients with the CBFP reaction, 35 patients were subjected to follow-up for five years. 26 out of 35 these patients were at a 1:1 initial RPR titer, 8 out of 35 patients were at a 1:2 initial RPR titer, and 1 out of 35 patients were at a 1:4 initial RPR titer. 30 subjects had their RPR seroreverted. In our opinion, additional CBFP research using a large sample population will contribute to the identification of additional underlying serious disorders that are not related to syphilis. Such results could be useful for the prediction and diagnosis of these diseases.