Ag85B Antigen in Bronchoalveolar Lavage Fluid: A Promising Biomarker for Tuberculosis Diagnosis

Objective: To assess the diagnostic performance of the Mycobacterium tuberculosis (MTB)‑secreted antigen Ag85B in bronchoalveolar lavage fluid (BALF) for pulmonary tuberculosis (PTB) and to explore its utility as a supplemental etiological test. Methods: We prospectively enrolled 104 PTB patients who underwent fibre‑optic bronchoscopy at the Affiliated Hospital of North Sichuan Medical College from May 2021 to July 2023, together with 72 non‑PTB controls who had pulmonary infections of non‑tuberculous origin. Ag85B concentrations in BALF were measured by enzyme‑linked immunosorbent assay (ELISA). Receiver‑operating‑characteristic (ROC) analysis established the optimal diagnostic cut‑off (maximum Youden index). Diagnostic performance indices were compared with acid‑fast bacillus (AFB) smear microscopy and the GeneXpert MTB/RIF assay. Results: BALF‑Ag85B levels were significantly higher in PTB patients than in non‑PTB controls (P < 0.001) and did not differ between bacteriologically confirmed (BC‑PTB) and clinically diagnosed (CD‑PTB) cases (P > 0.05). Sensitivities were 26.92 % for AFB smear, 56.73 % for GeneXpert MTB/RIF, and 64.42 % for Ag85B ELISA; the latter was significantly more sensitive than AFB smear (P < 0.001) and similar to GeneXpert MTB/RIF (P = 0.256). Specificities were 100 % for AFB smear and GeneXpert MTB/RIF versus 79.17 % for Ag85B ELISA (P < 0.001). Ag85B sensitivity was comparable between BC‑PTB (67.80 %) and CD‑PTB (60 %) groups (P = 0.411). Conclusion: ELISA detection of Ag85B in BALF provides a useful adjunct for PTB diagnosis, particularly in bacteriologically negative cases.