[The ratio of tuberculosis-specific antigen to phytohemagglutinin in T-SPOT assay in the diagnosis of active tuberculosis]

Objective: The aim of this study was to determine the performance of the ratio of tuberculosis-specific antigen (TBAg) to phytohemagglutinin (PHA) (TBAg/PHA ratio) in T-SPOT assay in the diagnosis of active tuberculosis (ATB). Methods: Between January 2014 and January 2017, 378 Mycobacterium tuberculosis (MTB) culture positive patients (268 cases of pulmonary tuberculosis, 110 extra-pulmonary tuberculosis) and 824 healthy individuals were recruited from Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology. T-SPOT assay was performed and TBAg/PHA ratio was calculated in all the participants. To validate the study, another group of 223 MTB culture positive TB patients with positive T-SPOT results were recruited from Guangzhou Chest Hospital between January 2017 and December 2017. This was a retrospective case-control study and differences between groups were analyzed using the Mann-Whitney U-test. Results: Of the 378 culture positive ATB patients, 344 patients had positive T-SPOT results. Of the 824 healthy individuals, 204 individuals had positive T-SPOT results. Using healthy individuals as the control group, the sensitivity and specificity of T-SPOT assay in the diagnosis of ATB were 91.0% (344/378) and 75.2% (620/824). Directly using T-SPOT results had a limited accuracy in distinguishing ATB from latent tuberculosis infection (LTBI). The area under the receiver operating characteristic (ROC) curve was between 0.7 and 0.8. However, a further calculation of the TBAg/PHA ratio showed a better performance than TBAg in distinguishing these two conditions, and the area under the ROC curve was 0.881 (95% CI: 0.853-0.909). If using the threshold value of 0.234, the sensitivity and specificity of the TBAg/PHA ratio in distinguishing ATB from LTBI were 69.5% (239/344) and 94.12% (192/204). The validation data showed that the performance of the TBAg/PHA ratio in distinguishing ATB from LTBI was also satisfactory, and the area under the ROC curve was 0.901 (95% CI: 0.872-0.931). Furthermore, the TBAg/PHA ratio had an important role in the diagnosis of extra-pulmonary tuberculosis. If using the threshold value of 0.234, the sensitivity and specificity of the TBAg/PHA ratio in the diagnosis of extra-pulmonary tuberculosis were 79.2% (76/96) and 94.1% (192/204). The area under the ROC curve was 0.932 (95% CI: 0.897-0.967). Conclusions: The TBAg/PHA ratio in T-SPOT assay was better than directly using T-SPOT results in distinguishing ATB from LTBI. This ratio also showed a potential use in the diagnosis of extra-pulmonary tuberculosis.