According to our grasping, through there were some publications on evaluating the utility of T-SPOT for diagnosis of TPE [9–10], the majority of subjects recruited small sample population(n < 100) for evaluation and the results was conflicting.Our study provided the largest cohort(TPE:non TPE = 397:145) so far, confirmed that the value of T-SPOT assay for diagnosis of TPE with high confidence, and providing specific reference suggestions for clinical application of T-SPOT.
In this study, all suspected PE participants had been consecutively, unselectedly enrolled, and all TPE patients had the definite diagnosis by bacteriological confirmation or positive outcome for anti-TB therapy, which highly reflecting the demographic epidemiological characteristics of tuberculosis-prone areas. The recruited samples covered from adolescents to elderly patients aging 90+, confirmed again that the TPE patients had the clinical characteristics of younger male, combining with higher ADA activity and higher T-SPOT response for PF and PB. It’s similar to other studies when IGRA are applied to PB, the unsatisfactory specificity was common as a result of inefficiency to discriminating active TB from latent TB infection, reversely the specificity of T-SPOT for PF is much higher with approximate sensitivity, thus the diagnostic cutoff obtained from PB is not available for PF; in our study the cut-off value derived from ROC analysis was higher than that in low prevalence areas,majority of which had taken as equal as PB cutoff in Europe,and some subjects obtained the cutoff by ROC analysis also is very low, such as 30SFCs/106 mononuclear cells in Londonand 300 SFCs/106 mononuclear cells in Korea,respectively, which was in line with the expect of high-burden settings low threshold would compromise specificity, therefore it’s crucial for tuberculosis-prone areas that to chose a proper threshold to apply PF T-SPOT.
Another biomarker ADA, a value of more than 40 IU/L in lymphocyte dominate PE carries a PPV of 98% in high TB endemic region[3–4]; while an retrospective analysis of assessing ADA in 1637 subjects confirmed less than 15 IU/L can get a NPV of 100%.As same as in our study, we found that recognizing > 40 IU/L as the solo indicator of TPE maybe not the most suitable, as it got the high specificity meanwhile sacrificing sensitivity, 35.5%(141/397) TPE patients’ ADA level was lower than 40 IU/L in this study, 29.8% of which had definite etiological basis. In addition, the utility of cutoff 22.4 IU/L derived from ROC analysis was better than > 40 IU/L(Youden index 0.729 vs. 0.641). However, comprehensively considering to the non-specific elevation of ADA level caused by the non tuberculosis inflammatory PE(particularly complicated parapneumonic effusions and empyemas) and lymphomas, the patients whose ADA more than 20 IU/L and less than 40 IU/L could be classified as ADA indeterminate appreciatively. Reversely, PF T-SPOT showed the excellent diagnostic utility between ADA indeterminate groups, whose accuracy is higher to 90.2%, predicted that the result of PF T-SPOT could be a considerable indicator for the highly-suspected TPE patients with indeterminate ADA.
Many previous studies indicated that[19–20], the performance of diagnosing TPE for patients aging more than 45yrs is still an open question, a recent study about only 4.65% of TPE subjects that main compose of elderly primarily had levels over 40 IU/L,our research fully demonstrated this phenomenon simultaneously. In addition, we observed that, the fuzzy boundary influenced by age on ADA is opportunely concentrated in ADA indeterminate groups.among the patients of ADA indeterminate, there were 85.7%(18/21), 73.7%(19/26) and 47.7%(31/65) patients aging more than 45yrs in non-, confirm- and probable- TPE group, respectively. while the superior performance of PF T-SPOT between ADA indeterminate groups may be explained by its steady at all age stages, besides the interference for ADA by other inflammatory etiologies. Still, ADA is a widely-use biomarker for screening TPE due to its simplicity, rapidity, and low finical costs,but above results proved that over-reliance on ADA differentiation may lead to missed diagnosis/misdiagnosis in clinical diagnosis, especially in indeterminate range. Meanwhile, PF T-SPOT as luck would fill this blank.
Besides age,we screened another two high risk factors that were significantly related to TPE, gender and BMI. There were barely 5 non TPE patients scoring 11(simultaneously fulfill three high risk factors:age < 45yrs,male and BMI < 22), it has directly demonstrated these clinical characters could be the effective reference index for discern TPE from other PEs. We often defined these patients as the high incidence population to TPE, also called as typical population. However, it’ s notably that the utility of ADA fluctuated distinctly by stratified analysis, and if not satisfied any one condition(defined as the unconventional population) it is inferior to that in T-SPOT assay. The unconventional population frequently are the focal points and have difficulties in diagnosing, then PF T-SPOT can provide powerful identification evidence for those.
There are several limitations on current study. First, The whole study were performed in a single center which specialized in TB. The geography, relative single control composition and etiological attribution error and/or bias was incalculable. Second, we got the optimal cut-off of PF T-SPOT.TB from ROC analysis in this training cohort, and its definite accuracy would need the further validation. Finally, 38.1% of the clinical diagnosis patients lack the etiological basis due to the objective factors such as no sputum or sputum unsatisfied with the detection standard, which may bias sputum detection rate.