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Introduction TB infection (TBI) comprises a spectrum of infection states that may be characterised using FDG PET-CT by assessing the presence and burden of metabolic activity and structural change (PMID: 38914470). We evaluated whether routinely collected peripheral blood immunological and haematological inflammatory markers can inform the burden of PET-CT-defined inflammation in household contacts with TBI.Methods We conducted a prospective cohort study of pulmonary TB contacts recruited between September 2021 and April 2024. Participants were recruited shortly after index case notification and underwent QuantiFERON-TB Gold Plus (QFT), full blood count, CRP, and PET-CT at baseline. QFT-positive contacts were prospectively followed for 2 years without receiving chemoprophylaxis. Monocyte-to-lymphocyte ratio (MLR), neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), CRP, and quantitative TB1, TB2, and TB2-TB1 IFN-γ responses were investigated for associations with PET-CT-defined inflammation.Results (see table 1).Thirty-one of 56 QFT-positive contacts analysed exhibited FDG uptake consistent with metabolically active TBI: 26 with intra-thoracic lymph node (ITLN) involvement and 14 with lung involvement. Eight progressed to TB, including two diagnosed within 3 months of the index notification. PET lung positivity at baseline was significantly associated with TB progression (χ2=19.4, p<0.001).TB1 and TB2 responses showed weak associations with the presence of ITLN positivity, but not with the extent of ITLN inflammation (SUVmax, total lesion glycolysis, data not shown). In contrast, MLR, NLR, PLR and TB2-TB1 did not associate with ITLN involvement, but did show moderate associations with the presence of PET lung positivity (AUCs: 0.81, 0.76, 0.70 and 0.69, respectively). PLR also correlated significantly with lung SUVmax (ρ= 0.55, p=0.04).MLR, NLR, and PLR were moderately associated with TB progression (AUCs: 0.75, 0.67 and 0.78, respectively). CRP did not demonstrate associations with any evaluated outcomes.Abstract M36 Table 1Associations between peripheral blood markers with PET-CT-defined inflammation and TB progression PET ITLN Positivity AUC (95% CI ) PET Lung Positivity AUC (95% CI ) Lung SUVmax ρ (p-value) TB Events AUC (95% CI ) Monocyte-to-lymphocyte ratio 0.63 (0.49 – 0.78) 0.81 (0.65 – 0.97) 0.34 (0.24) 0.75 (0.52 – 0.98) Neutrophil-to-lymphocyte ratio 0.56 (0.41 – 0.71) 0.76 (0.62 – 0.90) 0.46 (0.10) 0.67 (0.46 – 0.89) Platelet-to-lymphocyte ratio 0.54 (0.39 – 0.70) 0.70 (0.54 – 0.86) 0.55 (0.04) 0.78 (0.58 – 0.98) CRP 0.53 (0.37 – 0.68) 0.47 (0.23 – 0.67) 0.38 (0.22) 0.44 (0.21 – 0.67) QuantiFERON-TB Gold Plus TB1 0.66 (0.52 – 0.80) 0.51 (0.35 – 0.67) 0.17 (0.56) 0.57 (0.36 – 0.78) TB2 0.64 (0.49 – 0.78) 0.54 (0.39 – 0.70) 0.12 (0.69) 0.57 (0.36 – 0.78) TB2 – TB1 0.43 (0.27 – 0.59) 0.69 (0.53 – 0.84) 0.18 (0.54) 0.55 (0.33 – 0.78) Associations between peripheral blood markers with PET-CT-defined inflammation and TB progression were evaluated using the area under the receiver operating characteristic (ROC) curve (AUC). The relationship between peripheral blood makers and lung SUVmax was assessed using Spearman’s rank correlation coefficient (ρ).TB Antigen Tube 1 (TB1) detects CD4+ T-cell responses to TB antigens. TB Antigen Tube 2 (TB2) detects both CD4+ and CD8+ T-cell responses. TB1 and TB2 values were corrected for the Nil response.AUC: area under the curve; ITLN: intrathoracic lymph node; PET: positron emission tomography; SUVmax: maximum standardised uptake value; TB: tuberculosis; ρ: Spearman’s rank correlation coefficient, 95% CI: 95% confidence interval.Conclusions Peripheral blood inflammatory and immune markers are differentially associated with specific patterns of PET-defined inflammation in household contacts with TBI. Specifically, IFN-γ responses associate with the presence of ITLN activity, possibly indicative of a protective adaptive immune response. In contrast, haematological ratios associate with lung parenchymal inflammation that may be indicative of a failing protective immune response associated with an increased likelihood of TB progression.