New insights into the TB-IGRA: Impact of endotoxin in blood collection tubes on test results

OBJECTIVE: To investigate the impact of the collection tube as an external factor contributing to high background signals in the tuberculosis-interferon gamma release assay (TB-IGRA), with a specific focus on the influence of endotoxin contamination in blood collection tubes. METHODS: We conducted a retrospective analysis of TB-IGRA test results and clinical data from 334 patients. Patients were stratified into high and low background value groups based on median IFN-γ levels in the negative control wells of the TB-IGRA test. Additionally, blood samples from 25 healthy subjects were collected using two brands of blood collection tubes- Brand 1 and Brand 2-to assess differences in background signal intensity. Endotoxin levels in both tube types were measured, and graded concentrations of exogenous endotoxin were added in vitro to blood samples to evaluate the impact on IFN-γ background levels in the TB-IGRA test. RESULTS: Among clinical samples, 31.44% (105/334) exhibited elevated background values (IFN-γ levels≥0.438 IU/mL). Patients in the high background signal group had higher levels of white blood cell, neutrophils, lymphocytes, monocytes, and C-reactive protein (P < 0.05). Notably, the use of Brand 1 tubes was associated with high background values (P < 0.001). In healthy subjects, the mean IFN-γ value obtained with Brand 1 tubes was significantly higher than that with Brand 2 tubes (0.407 vs. 0.180 IU/mL, P = 0.038). In vitro experiments demonstrated a dose-dependent increase in IFN-γ background levels following endotoxin exposure. CONCLUSION: Endotoxin contamination in blood collection tubes is a significant factor contributing to elevated background signals in TB-IGRA tests. Clinical laboratories should carefully consider the endotoxin control standards of blood collection tubes for use.