To analyze the value of T-cell spot of tuberculosis (T-SPOT.TB) combined with serum 25-dihydroxycholecalciferol D₃ [25-(OH)D₃] and antimicrobial peptide LL-37 in the diagnosis of pulmonary tuberculosis (PTB) with diabetes mellitus (DM).

The clinical data of 340 patients with suspected PTB who underwent T-SPOT.TB and detection of serum 25-(OH)D₃ and LL-37 in our hospital from January 2015 to December 2018 were retrospectively analyzed. A total of 121 patients with only PTB were enrolled in the PTB group, while 97 patients with PTB and DM were enrolled in the PTB-DM group. And the 122 patients without PTB infection or DM were included in the control group. The levels of specific antigen culture filtrate protein 10 (CFP10), the number of spot forming cells (SFCs) in the 6000 early secretion antigen target (ESAT-6), and the levels of serum 25-(OH)D₃ and LL-37 were compared among the three groups. The values of T-SPOT.TB combined with serum 25-(OH)D₃ and LL-37 in the diagnosis of PTB with DM were analyzed through drawing the receiver operating characteristic curve (ROC curve).

①The total sensitivity, specificity, accuracy, positive predictive value, negative predictive value and kappa value of T-SPOT.TB for the diagnosis of PTB were 76.6%, 77.0%, 76.8%, 85.6%, 64.8% and 0.515, respectively, indicating general consistency. ②Antigen CFP10 and SFCs in the antigen ESAT-6 were the highest in the PTB-DM group, followed by the PTB group and the control group (P<0.05). The areas under the curves (AUCs) of antigen CFP10 and SFCs in the antigen ESAT-6 were 0.871 and 0.872, respectively. The cut-off values were 16.13 SFCs /2.5×10⁵ PBMC and 14.80 SFCs/2.5×10⁵ PBMC, respectively. The AUC, sensitivity and specificity of the combined diagnosis were 0.931, 83.5% and 87.2%, respectively. ③The level of serum 25-(OH)D₃ was the lowest in the PTB-DM group, followed by the PTB group and the control group, while the level of LL-37 was the highest in the PTB-DM group, followed by the PTB group and the control group (all P<0.05). The AUCs were 0.630 and 0.653, respectively. The cut-off values were 18.06 ng/ml and 59.71 ng/ml, respectively. The sensitivity and specificity for the diagnosis of PTB with DM were (90.7%, 45.4%) and (33.3%, 91.4%), respectively. ④The AUC of T-SPOT.TB combined with 25-(OH)D₃ and LL37 was the largest (0.933), with the sensitivity and specificity of 79.4% and 92.2%, respectively.

Single T-SPOT.TB diagnosis is not effective for PTB with DM. T-SPOT.TB combining with serum 25-(OH)D₃ and LL-37 can improve the diagnostic efficiency, which provides references for the clinical treatment of the patients with PTB and DM.