Background: Ratios of different immune cell populations (i.e., monocyte-to-lymphocyte, neutrophil-to-lymphocyte, and platelet-to-lymphocyte ratios) have been studied as a means of predicting future tuberculosis (TB) disease risk or to assist in the diagnosis of incident TB disease. No studies to-date, however, have evaluated the potential of these ratios to predict or assist in the diagnosis of incident TB infection - the first step in the natural history of TB disease.
Methods: In this prospective study, we evaluated the complete blood count (CBC)-derived metrics of monocyte-to-lymphocyte ratio (MLR), neutrophil-to-lymphocyte ratio (NLR), and platelet-to-lymphocyte ratio (PLR) as predictors of future TB infection risk or aids in the diagnosis of TB infection among 145 Tanzanian adolescents enrolled in the DAR-901 vaccine trial, using paired CBCs and interferon-gamma release assays (IGRAs) obtained at 0, 60 and 720 days after study enrollment.
Results: At baseline, there were no significant differences between study participants who remained persistently IGRA negative throughout the study period and those who subsequently converted to IGRA positive with respect to MLR (0.18 vs 0.17, p = 0.10), NLR (0.88 vs 1.02, p = 0.08), or PLR (115 vs 120, p = 0.28). Similarly, no significant differences were noted with respect to MLR, NLR, and PLR between IGRA converters and time-matched negative controls at the time of IGRA conversion. With respect to other blood cell measures, however, there were modest but significant differences between IGRA negatives and IGRA converters with respect to red blood cell count (4.8 vs 4.6 × 106 cells/mcL, p = 0.008), hemoglobin (12.6 vs 12.3 g/dL, p = 0.01), and hematocrit (38.8 vs 37.8%, p = 0.005).
Conclusions: In contrast to prior studies that have suggested that the ratios of different immune cell populations are associated with development of TB disease, our present findings do not demonstrate an association between these ratios and the development of TB infection. However, decreased red blood cell measures were associated with the subsequent development of TB infection, suggesting either that dysregulation of iron metabolism may play a role in TB pathogenesis or that following TB infection, iron dysregulation may precede IGRA positivity.
Trial registration: Clinicaltrials.gov NCT02712424 . Date of registration: March 14, 2016.
Keywords: Complete blood count; Mycobacterium tuberculosis; Red blood cells; TB vaccine; Tuberculosis infection; White blood cells.