Background: The tuberculin skin test (TST) identifies individuals at high risk of developing tuberculosis (TB) but poses many challenges. The blood monocyte-to-lymphocyte ratio (MLR) could be an alternative, as extremes in MLR have been associated with increased risk of TB disease.
Methods: At a primary care clinic in Johannesburg, a differential white blood cell count and TST was performed in adults starting antiretroviral treatment (ART) without symptoms suggestive of active TB.
Results: Of 259 participants, 171 had valid results of whom 30% (51/171) were TST positive and the median MLR was 0.18 (IQR 0.13-0.28). The MLR distribution differed between CD4 count categories (p < 0.01), with a broader range of values in TST negative participants with a low CD4 count (≤ 250 cells/mm3), likely reflecting HIV immunosuppression. MLR was associated with a positive TST (OR 0.78 per 0.1 increase, 95% CI 0.59, 0.97) in bivariate analysis but not in multivariate regression analysis (aOR 0.83 for every 0.1 increase, 95% CI 0.60, 1.08).
Conclusion: In ART-naïve adults without symptoms suggestive of active TB, MLR was not independently associated with TST positivity and is thus unlikely to be a useful alternative to TST. Future research should focus on development of a cheap, simple and accurate biomarker to identify those people benefiting most from preventive TB therapy.