Abstract
BackgroundInterferon-gamma (IFN-γ) Release Assays (IGRA) are more specific than the tuberculosis skin test (TST) in the diagnosis of latent tuberculosis (TB) infection (LTBI). We present the performance of the QuantiFERON®-TB Gold In-tube (QFT-TB) assay as diagnostic test and during follow-up of preventive TB therapy in outpatients from a TB low-endemic country.Methods481 persons with suspected TB infection were tested with QFT-TB. Thoracic X-ray and sputum samples were performed and a questionnaire concerning risk factors for TB was filled. Three months of isoniazid and rifampicin were given to patients with LTBI and QFT-TB tests were performed after three and 15 months.ResultsThe QFT-TB test was positive in 30.8% (148/481) of the total, in 66.9% (111/166) of persons with origin from a TB endemic country, in 71.4% (20/28) previously treated for TB and in 100% (15/15) of those diagnosed with active TB with no inconclusive results. The QFT-TB test was more frequently positive in those with TST ≥ 15 mm (47.5%) compared to TST 11-14 mm (21.3%) and TST 6-10 mm (10.5%), (p < 0.001). Origin from a TB endemic country (OR 6.82, 95% CI 1.73-26.82), recent stay in a TB endemic country (OR 1.32, 95% CI 1.09-1.59), duration of TB exposure (OR 1.59, 95% CI 1.14-2.22) and previous TB disease (OR 11.60, 95% CI 2.02-66.73) were all independently associated with a positive QFT-TB test. After preventive therapy, 35/40 (87.5%) and 22/26 (84.6%) were still QFT-TB positive after three and 15 months, respectively. IFN-γ responses were comparable at start (mean 6.13 IU/ml ± SD 3.99) and after three months (mean 5.65 IU/ml ± SD 3.66) and 15 months (mean 5.65 IU/ml ± SD 4.14), (p > 0.05).ConclusionOnly one third of those with suspected TB infection had a positive QFT-TB test. Recent immigration from TB endemic countries and long duration of exposure are risk factors for a positive QFT-TB test and these groups should be targeted through screening. Since most patients remained QFT-TB positive after therapy, the test should not be used to monitor the effect of preventive therapy. Prospective studies are needed in order to determine the usefulness of IGRA tests during therapy.
Highlights
Interferon-gamma (IFN-g) Release Assays (IGRA) are more specific than the tuberculosis skin test (TST)in the diagnosis of latent tuberculosis (TB) infection (LTBI)
The QuantiFERON®-TBGold In-tube (QFT-TB) test was positive in 30.8% (148/481) of the total, in 66.9% (111/166) of persons with origin from a TB endemic country, in 71.4% (20/28) previously treated for TB and in 100% (15/15) of those diagnosed with active TB with no inconclusive results
Recent stay in a TB endemic country expressed as years since last stay was associated with a positive QFT-TB test, while visit to a TB endemic country was not
Summary
Interferon-gamma (IFN-g) Release Assays (IGRA) are more specific than the tuberculosis skin test (TST)in the diagnosis of latent tuberculosis (TB) infection (LTBI). Interferon-gamma (IFN-g) Release Assays (IGRA) are more specific than the tuberculosis skin test (TST). Gold In-tube (QFT-TB) assay as diagnostic test and during follow-up of preventive TB therapy in outpatients from a TB low-endemic country. The Interferon-gamma (IFN-g) Release Assays (IGRA), offering better specificity in the diagnosis of latent tuberculosis (TB) infection (LTBI) than the tuberculosis skin test (TST) [1,2,3,4,5,6], are recommended in many national TB programs in low-endemic countries [6,7,8]. There is limited data concerning the usefulness of the IGRA tests to identify those individuals with LTBI who are at most risk for developing active disease and most likely to benefit from preventive therapy [20]
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