Diagnosis of latent tuberculosis infection in healthy young adults in a country with high tuberculosis burden and BCG vaccination at birth

Abstract

Background: One third of the world's population is thought to have latent tuberculosis infection (LTBI) with the potential for subsequent reactivation of disease. To better characterize this important population, studies comparing Tuberculin Skin Test (TST) and the new interferon-γ release assays including QuantiFERON®-TB Gold In-Tube (QFT-GIT) have been conducted in different parts of the world, but most of these have been in countries with a low incidence of tuberculosis (TB). The aim of this study was therefore to evaluate the use of QFT-GIT assay as compared with TST in the diagnosis of LTBI in Ethiopia, a country with a high burden of TB and routine BCG vaccination at birth.

Methods: Healthy medical and paramedical male students at the Faculty of Medicine, Addis Ababa University, Ethiopia were enrolled into the study from December 2008 to February 2009. The TST and QFTG-IT assay were performed using standard methods.

Results: The mean age of the study participants was 20.9 years. From a total of 107 study participants, 46.7% (95%CI: 37.0% to 56.6%) had a positive TST result (TST≥10 mm), 43.9% (95%CI: 34.3% to 53.9%) had a positive QFT-GIT assay result and 44.9% (95%CI: 35.2% to 54.8%) had BCG scar. There was strong agreement between TST (TST ≥10mm) and QFT-GIT assay (Kappa = 0.83, p value = 0.000).

Conclusion: The TST and QFT-GIT assay show similar efficacy for the diagnosis of LTBI in healthy young adults residing in Ethiopia, a country with high TB incidence.

Figure 1

Scatter plot of TST measurement and QFT-GIT assay (IU/ml) results. Tuberculin skin test (TST) was performed by injecting 0.1 ml Tuberculin PPD RT 23 intradermally on the ventral aspect of the left forearm. After 48-72 hours, the transverse diameter of the skin induration was measured using a plastic ruler graduated in millimeters. A positive TST result was defined as an induration of ≥10mm. QuantiFERON®-TB Gold In-Tube (QFT-GIT) assay was performed according to the manufacturer’s instructions. The QFT-GIT (IU/ml) raw data was analyzed by using QuantiFERON®-TB Gold IT Analysis Software and reported as Negative, Positive and Indeterminate. In this figure, the correlation between the two latent tuberculosis infection (LTBI) test results, TST in mm and QFT-GIT in IU/ml, was assessed by spearman correlation; Spearman correlation coefficient = 0.81, p<0.05.