Clinical Utility of Monocyte-to-Lymphocyte Ratio and Prognostic Nutritional Index in Diagnosing Smear-Negative Pulmonary Tuberculosis with Negative IGRA Results

Yan Liu,1– 3 Mei Li,2 Yunyun Ding,2 Yong Gao,2 Tuantuan Li,2 Xiaowu Wang,2 Meijuan Zheng1 1Department of Clinical Laboratory, First Affiliated Hospital of Anhui Medical University, Hefei, People’s Republic of China; 2Department of Clinical Laboratory, The Second People’s Hospital of Fuyang City, Fuyang, People’s Republic of China; 3Department of Clinical Laboratory, Fuyang Infection Disease Clinical College of Anhui Medical University, Fuyang, People’s Republic of ChinaCorrespondence: Xiaowu Wang, Department of Clinical Laboratory, The Second People’s Hospital of Fuyang City, Fuyang, People’s Republic of China, Email wangxiaowu19880218@126.com Meijuan Zheng, Department of Clinical Laboratory, First Affiliated Hospital of Anhui Medical University, Hefei, People’s Republic of China, Email mjzheng@mail.ustc.edu.cnObjective: To evaluate the diagnostic usefulness of the monocyte-to-lymphocyte ratio (MLR) and the prognostic nutritional index (PNI) in smear-negative pulmonary tuberculosis (SN-PTB) patients with a negative interferon-γ release assay (IGRA) result.Methods: Between January 2021 to December 2024, 548 consecutive patients suspected of having SN-PTB were enrolled at the Second People’s Hospital of Fuyang. After exclusion, 276 patients with SN-PTB (SN-PTB group) and 272 with non-tuberculous (Non-TB) pulmonary infection (Non-TB group) were retrospectively analyzed. Laboratory parameters—including albumin (ALB), pre-albumin (PALB), C-reactive protein (CRP), fibrinogen (FIB), IGRA result, T-cell subsets (CD3+, CD4+, CD8+), CD4+/CD8+ ratio, MLR, and neutrophil-to-lymphocyte ratio (NLR)—were compared between groups. Patients were further stratified by IGRA status (positive vs negative). Receiver-operating characteristic (ROC) curves were constructed to assess the diagnostic performance of individual and combined indices.Results: In the IGRA-negative subgroup (SN-PTB group: n = 25; Non-TB group: n = 184), MLR was significantly higher in the SN-PTB group (p = 0.008) and PNI was significantly lower (p < 0.001). No significant differences were observed in ALB, FIB, CRP, PALB, or T-cell subsets in this subgroup. The combination of MLR and PNI—using a logistic regression-derived score—achieved the best discriminatory power, with an area under the ROC curve (AUROC) of 0.718 (95% CI: 0.656– 0.774), sensitivity of 60.00%, and specificity of 82.38% at the optimal cut-off value.Conclusion: MLR, PNI and their combination offer moderate diagnostic value for SN-PTB when the IGRA result is negative; the combined MLR+PNI index performs better than either marker alone. However, these findings should be interpreted with caution due to the limited sample size in the IGRA-negative subgroup, and further validation in larger cohorts is warranted.Keywords: smear-negative pulmonary tuberculosis, interferon-gamma release assay, MLR, PNI, diagnostic value