Epidemiological analysis of sexually transmitted infections pathogens and related microorganisms in South Korea: coinfection patterns and pathogen correlations

Sexually transmitted infections (STIs) affect hundreds of millions of people worldwide each year and remain a major public health concern. The adoption of multiplex PCR, which allows simultaneous detection of multiple pathogens, has led to the accumulation of large-scale data on STI coinfections. In this study, we analyzed multiplex PCR results for 12 microorganisms included in a commercial STI testing panel to characterize epidemiological patterns in South Korea and to quantitatively evaluate pathogen–pathogen coinfection relationships using lift index and phi coefficient. We retrospectively analyzed multiplex PCR results for 12 microorganisms included in an STI testing panel — Neisseria gonorrhoeae (N. gonorrhoeae), Chlamydia trachomatis (C. trachomatis), Mycoplasma hominis (M. hominis), Trichomonas vaginalis (T. vaginalis), Ureaplasma urealyticum (U. urealyticum), Mycoplasma genitalium (M. genitalium), Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2), Gardnerella vaginalis (G. vaginalis), Treponema pallidum (T. pallidum), Candida albicans (C. albicans), and Ureaplasma parvum (U. parvum)— from individuals aged 18–90 years tested at Eone Laboratories between January 2023 and December 2024. DNA was extracted using the Nextractor® NX-48 system, and PCR was performed with the ElsiQ STI-12 Detection Kit. Pathogen positivity rates were assessed by sex, age, and specimen type. Association rule mining was applied to evaluate coinfection patterns using the lift index and phi coefficient, and the results were visualized as heatmaps. A total of 1,105,261 samples were analyzed, with an overall positivity rate of 8.67% among all individual pathogen-specific tests. At the specimen level, 43.6% were negative, 26.1% were positive for a single pathogen, and 30.4% had coinfections. Compared with males, females presented a greater rate of coinfections. For most pathogens, positivity rates were highest in individuals under the age of 30 years and tended to decrease with age. For some pathogens, a bimodal pattern of positivity was observed in females, with a second increase occurring around the age of 50. Among female samples, vaginal swabs presented higher positivity rates than did urine samples for all pathogens. Analysis of coinfection patterns revealed that the lift index tended to be greater for pathogen pairs with low individual positivity rates, whereas the phi coefficient was relatively higher for pathogens with higher positivity rates. Pathogen pairs that presented a lift index ≥ 3 in both male and female samples included N. gonorrhoeae–C. trachomatis, M. hominis–T. vaginalis, T. pallidum–T. vaginalis, and T. pallidum–C. trachomatis, whereas U. parvum–M. hominis and U. urealyticum–M. hominis presented phi coefficients ≥ 0.1 in both sexes. This study provides quantitative insights into the epidemiological characteristics and inter-pathogen coexistence of STI-related microorganisms in South Korea using multiplex PCR–derived data and association metrics, including the lift index and phi coefficient. These findings may serve as foundational data for refining future STI surveillance strategies and for further investigations into the mechanisms underlying coinfections. Not applicable.