Northern delights

In assessing the performance of QuantiFERON these Norwegian researchers explain the reasons for using QuantiFERON (see below). Importantly, this study was

a longitudinal study performed in a regular outpatient TB clinic in a TB low-endemic country. In contrast to a number of studies focusing on particular populations, we investigated the performance of the QFT-TB test in various risk populations referred according to present national guidelines and thus reflecting the clinical situation at a large size Norwegian hospital. We further present data of IFN-γ responses during preventive therapy of latent TB.

and it was found that

The performance of the QFT-TB test was good in this clinical setting.

Background

The Interferon-gamma (IFN-γ) Release Assays (IGRA), offering better specificity in the diagnosis of latent tuberculosis (TB) infection (LTBI) than the tuberculosis skin test (TST) [1-6], are now recommended in many national TB programs in low-endemic countries [6-8]. 

There are two commercial assays available and although T-SPOT.TB^® seems to give higher sensitivities in immunocompromised patients [9], the QuantiFERON^®-TB Gold In-tube (QFT-TB) is often the test of choice in the clinical setting due to easier logistics when processing samples. In a meta-analysis the pooled sensitivity for QFT-TB was 70-78% and the specificity was 99% among non-BCG-vaccinated and 96% among BCG-vaccinated persons. The authors conclude that the IGRAs have excellent specificity that is unaffected by BCG vaccination [6].

Many studies have focused on the performance of the IGRA tests in active TB [4] or in certain risk groups as TB contacts [3,5], health-care workers [10] or in patients treated with tumor necrosis factor-alfa (TNF-α) inhibitors [11,12]. Fewer studies have been performed in outpatient clinical settings including individuals referred for various reasons according to clinical practise and national guidelines [9]. All studies are limited by the lack of a diagnostic gold standard for LTBI. The effect of preventive therapy on IFN-γ responses [13-16] and the cost-effectiveness of the IGRA tests on this patient population are still controversial [17,18]. Diel et al. reported that QFT-TB is a more accurate indicator of progression to active TB than TST [19]. Still, 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 therefore most likely to benefit from preventive therapy [20].

Norway is a TB low-endemic country and the Norwegian population has until 2009 been BCG vaccinated at the age of fourteen, whereas the immigrant groups are often vaccinated as infants. Further, non-tuberculous mycobacteria (NTM) infections are also quite common [21,22]. These factors cause difficulties in diagnosing LTBI since the specificity of the TST test is low and variable in the BCG-vaccinated population. Immigration from TB high-endemic countries and increased global travelling with possible TB exposure challenge the epidemic situation [23]. Thus, the various groups demonstrating a positive TST test are very heterogeneous and more reliable diagnostic tools are needed to identify those with LTBI in order to offer proper preventive therapy and follow-up.

We performed a study to evaluate the usefulness of the QFT-TB test in the diagnosis of active and latent TB in a typical outpatient clinic in a TB low-endemic country. We analysed the various risk factors for a positive QFT-TB test. Further, we studied the reversion rates of the QFT-TB test and the IFN-γ responses right after and one year after ended preventive TB therapy in QFT-TB positive individuals.