Abstract: 【Abstract】 Objective To investigate the appropriate signal-to-cutoff （S/CO） value ranges by chemiluminescent microparticle immunoassay （CMIA） required for the retest of syphilis-specific antibodies. Methods This study was a retrospective case series study. Data were collected from patients who underwent syphilis antibody testing at the Hospital for Skin Diseases, Chinese Academy of Medical Sciences between July 2020 and June 2024. The Treponema pallidum particle agglutination assay （TPPA） served as the confirmatory test. The positive predictive value and negative predictive value of CMIA for the detection of syphilis antibodies across different ranges of S/CO values were analyzed to determine the appropriate retest range. In addition, the clinical characteristics of patients with discordant results between CMIA and TPPA were analyzed and summarized. Results A total of 1 735 samples were tested for syphilis-specific antibodies using CMIA, which were collected from 1 153 males and 582 females, aged 1 - 95 （46.29 ± 17.46） years. Among them, 160 samples were CMIA-negative （S/CO < 1）, and all were confirmed negative by TPPA; of the 1 575 CMIA-positive samples （S/CO ≥ 1）, 1 485 （94.29%） were TPPA-positive, while 90 were TPPA-negative. Based on the S/CO values, the 1 575 CMIA-positive samples were divided into 6 groups according to the following ranges: 1 - < 3, 3 - < 5, 5 - < 7, 7 - < 9, 9 - < 11, and ≥ 11. The TPPA positivity rates increased progressively with increasing S/CO values. As the S/CO range increased from 1 - < 3 to 7 - < 9, the positive predictive value of CMIA increased from 36.08% to 96.88%, reaching 100.00% when S/CO ≥ 9. A total of 90 cases （5.19%） showed discordant results between CMIA and TPPA, all of which were CMIA-positive （S/CO values: 1.004 - 8.137） but TPPA-negative. Conclusions The retest of syphilis-specific antibodies may be required when the S/CO values by CMIA are 1 - < 9, while TPPA retest may not be required when the S/CO values are ≥ 9 or < 1. For cases with CMIA-positive but TPPA-negative results, comprehensive evaluation should be performed in conjunction with clinical manifestations and epidemiological history.

Key words: Syphilis, Syphilis serodiagnosis, Chemiluminescent microparticle immunoassay, Treponema pallidum particle agglutination