Clinical Utility of Monocyte-to-Lymphocyte Ratio and Prognostic Nutritional Index in Diagnosing Smear-Negative...

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.

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.

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.

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.