Syphilis-positive and false-positive trends among US blood donors, 2013-2023

US blood donors are tested for syphilis because the bacterial agent is transfusion transmissible. Here we describe trends over an 11-year period of donations positive for recent and past syphilis infections, and donations classified as syphilis false positive (FP).

Data from January 1, 2013, to December 31, 2023 (11 years) were compiled for all American Red Cross blood donations to evaluate demographics/characteristics and longitudinal trends in donors testing syphilis reactive/positive. The prevalence of recent, past, and total (combination of recent/past) infections were evaluated, along with syphilis FP donations. Classification was based on known serological methods using treponemal and non-treponemal tests.

Approximately 53 million donations were included with 10,365 total syphilis infections (3463 recent and 6902 past; 0.02% total) and 48,719 FP (0.09%). Donor demographics, characteristics, and HBV/HCV/HIV nucleic acid testing reactivity differed among syphilis-positive and FP compared with nonreactive donations. Donors with a FP donation had high rates of subsequent donations testing syphilis positive (0.3%) and FP (7.7%). Median time from first FP to subsequent FP was observed to be 3.6 months. Recent infections increased over the 11 years, with past infections peaking in 2014 followed by increases in 2021-2023 paralleling recent infections. Repeating cycles of seasonal FP spikes occurred in the fall of 2013 and from 2017 to 2022 corresponding with vaccine administrations.

Syphilis infections in blood donors are trending upward, paralleling US population trends. Syphilis FP donors had high rates of subsequent FP and unexplained syphilis-positive donations. Seasonal syphilis FP spikes occurred for most years but are trending downwards.