The diagnostic value and performance evaluation of five serological tests for the detection of Treponema pallidum

Syphilis is caused by the bacterium Treponema pallidum (TP). The aim of this study was to establish a clinical approach for serodiagnosis of syphilis by evaluating the performance and diagnostic value of five serological tests for the detection of TP.

Five tests were used to test the serum from syphilis patients and control patients, namely rapid plasma reagin (RPR) test, toluidine red unheated serum test (TRUST), TP passive particle agglutination assay (TPPA), TP-specific enzyme-linked immunosorbent assay (TP-ELISA), and TP-specific chemiluminescent immunoassay (TP-CMIA).

The sensitivity and diagnostic efficiency of TPPA (96.25%/98.38%), TP-ELISA (100%/95.41%), and TP-CMIA (100%/94.86%) were significantly higher than that of RPR (73.13%/86.22%) and TRUST (73.75%/86.49%) (P < 0.05). The minimum detectable concentrations for the five tests were 30 mIU/ml, 20 mIU/ml, 15 mIU/ml, 150 mIU/ml, and 150 mIU/ml, respectively. According to receiver operating characteristic (ROC) curve, the optimal cut-off values for syphilis diagnosis by TP-CMIA and TP-ELISA were 2.2 and 2.0 S/CO (where S/CO = Sample/calibrator cut off), and the area under the ROC curve (AUC) were 0.998 for TP-CMIA and 0.999 for TP-ELISA. The titers/positive rates for RPR and TRUST dropped from 1:4 (100%) to 1:1 (23.3%) (both P < 0.05) after treatment. However, there were no significant differences when we compared the positive rate of syphilis patients before and after treatment by TPPA, TP-ELISA, and TP-CMIA.

Treponemal tests, such as TPPA, TP-ELISA, and TP-CMIA, are recommended for clinical routine screening of syphilis. However, nontreponemal tests, for example, RPR and TRUST, perform better in therapy response assessment. Serological test should be tailored to respective facilities and clinical demands.