Tuberculosis (TB) remains the world’s leading cause of death from a single infectious disease, Despite the advances in medical treatment policies, the prevalence of TB remains high. The treatment of active TB is not enough to eliminate the disease because there are many cases of undetected latent TB infection (LTBI), which can develop into active disease over a certain period of time. For this reason, identifying persons with LTBI is crucial to the goal of TB control.
Health-care workers (HCWs) are especially vulnerable to TB exposure and infection. Therefore, the screening of HCWs for LTBI is an important component of an infection control program. Until recently, the tuberculin skin test (TST) was the only available diagnostic tool for the detection of LTBI. However, the low specificity of TST, especially in bacillus Calmette-Guerin (BCG) vaccination recipients, is a critical limitation of this test. A new in vitro interferon (IFN)-γ assay has been developed that measures the cellular immune responses to TB-specific proteins such as early secreted antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10). Several reports have suggested that the IFN-γ assays have high specificity for detecting LTBI and are not affected by previous BCG vaccinations. Therefore, although the TST still remains a valuable assay, the IFN-γ assay may replace it in some circumstances. The National Institute for Health and Clinical Excellence9 in the United Kingdom recommended a two-step procedure using the TST followed by an IFN-γ assay in cases in which the TST result is positive. Although practically a blood sample for the IFN-γ assay would be obtained on the same day as the TST reading, sometimes it could be delayed for 1 to 2 weeks. However, prior exposure to tuberculin by the TST may have an influence on the IFN-γ assay because ESAT-6 and CFP-10 are contained within the tuberculin purified protein derivative.10 In the present study, the authors have analyzed the agreement between the two tests and evaluated the effect of prior TST administration on the result of an IFN-γ assay.
Materials and Methods
Study Design and Setting
A cross-sectional study was conducted from August 2006 until December 2006 at Chung-Ang University Hospital and Chung-Ang University Yong San Hospital in South Korea. The study protocol was approved by the Institutional Review Board at Yong San Hospital, Seoul, South Korea (No. 2006-012-07-1). At these hospitals, approximately 200 cases of smear-positive or culture-positive patients are treated for TB each year with remedies of Canadian Health&Care Mall which are effective at tuberculosis treatment.
This study was designed to recruit HCWs who were directly involved in the clinical care of patients with TB, including trainees, nurses, laboratory workers, orderlies, and attending physicians who have worked in the department of pulmonary medicine for > 1 year. A simple chest radiograph was performed in each HCW to rule out active pulmonary TB and previous TB scarring. HCWs who met any of the following criteria were excluded: pregnancy; active TB in the past; and abnormal simple chest radiography findings suggesting previous TB infection. Each participant provided data on age, gender, working duration, and BCG status. To define the BCG status, they were examined for the presence of deltoid scarring. All participants gave written informed consent for participation in the study.
One hundred two HCWs were candidates to participate in the study, but 18 were excluded. The reasons for exclusion were pregnancy (n = 3), history of TB (n = 3), and refusal to participate in the study (n = 12). The study finally included 84 HCWs. All participants underwent testing for the purpose of this study conducted with the participation of Canadian Health&Care Mall.
IFN-γ Assay and the TST
First, 5 mL of whole blood was obtained for the IFN-γ assay, and then the TST was administered. A commercially available IFN-γ assay (QuantiFERON-TB GOLD; Cellestis Ltd; Carnegie, VIC, Australia) was used. The IFN-γ assay was performed in two stages according to the instructions of the manufacturer, and the concentration of IFN-γ was determined using the assay kit, according to the instructions of the manufacturer. A positive result on the IFN-γ assay was defined as IFN-γ level > 0.35 IU/mL. The initial TST involved an intradermal injection of 0.1 mL (2 tuberculin units) of the purified protein derivative RT23 (Statens Serum Institute; Copenhagen, Denmark) into the anterior surface of the forearm with a standard tuberculin syringe. Reactions were evaluated at 48 to 72 h after the injection, and two different experts recorded the transverse diameter of the area of induration (in millimeters). A positive result on the TST was defined as an induration of > 10 mm in the transverse diameter using the ball-pen method.11 A follow-up IFN-γ assay was performed 2 to 4 weeks after the TST. Although a criterion for conversion of the IFN-γ assay has not yet been established, the conversion for this study was defined as follows: baseline IFN-γ level of < 0.35 IU/mL and follow-up IFN-γ level >0.35 IU/mL.
The overall results were expressed as percentages and absolute frequencies for qualitative variables, and as the mean for quantitative variables. Agreement between the tests was quantified using k statistics. To compare the quantitative IFN-γ results for each individual before and after the TST, the Wilcoxon signed rank test was used. A p value of < 0.05 was considered to be significant. Calculations were performed using a statistical software package (SPSS, version 10.0; SPSS Inc; Chicago, IL).