The diagnosis of tuberculosis (TB) infection relied solely on the tuberculin skin test (TST) until a decade ago when in vitro immunodiagnostic tests, known as interferon-gamma release assays (IGRAs), became available.

The TST, the reference method for the screening of people at risk, consists of the intradermal injection of PPD-S (Seibert's purified protein derivative), or the PPD-RT23 equivalent in Spain. The PPD contains a mixture of more than 200 antigens that are shared by mycobacteria other than Mycobacterium tuberculosis, and includes the vaccine strain of Mycobacterium bovis bacillus Calmette-Guérin (BCG) and most non-tuberculous mycobacteria (NTM). Therefore, people sensitized by previous exposure to NTM or the BCG vaccine may respond to a TST. Another major limitation is the loss of sensitivity of the test in certain groups, such as immunosuppressed patients and young children. It should also be remembered that a negative TST obtained early after the onset of infection with M. tuberculosis does not exclude infection because the test takes up to eight weeks to emerge as positive. This interval is usually referred to as the “window period.”

The IGRA tests are based on the in vitro quantification of the cellular immune response. IGRAs detect interferon-gamma (IFN-γ) release by sensitized T cells after stimulation with specific M. tuberculosis antigens. The two main antigens used are the 6-kD M. tuberculosis early-secreted antigenic target protein (ESAT-6) and the 10-kD culture filtrate protein (CFP-10), encoded in the region of difference 1 (RD1), which is present in M. tuberculosis but not in BCG nor in most NTM. This new technology was rapidly adapted from initial in-house methods to the two commercially available assays: the QuantiFERON®-TB Gold assay (QFT-G) (Qiagen, Hilden, Germany) and the T-SPOT®.TB assay (Oxford Immunotec, Oxford, UK). Like the TST, IGRAs have a window period of conversion after exposure to M. tuberculosis, but the duration of this window has not been clearly determined.

Currently there is no consensus on the use of IGRAs in different clinical scenarios and settings. Despite the abundance of literature about IGRAs, there is a scarcity of good quality evidence evaluating important patient outcomes in given populations. This, in turn, is reflected in the inconsistency of the recommendations of official organizations and scientific societies, which largely rely on expert opinion. Moreover, recommendations that are suitable for one area or country may not be appropriate for another. To this end, two consensus documents were produced in 2008 and 2010 on the diagnosis and treatment of TB in adults and children, respectively, on the use of IGRAs in Spain. Therefore, there was a need for updated recommendations, based on the best available evidence and formulated according to the modern methodology for the elaboration of guidelines.

The current document provides evidence-based guidance to the specialist and primary health-care providers and public health authorities on the use of IGRAs for diagnosing TB infection in both immunocompetent and immunosuppressed adults and children of any age at risk for or suspected of having active TB. The final objective of the document is to minimize the uncertainty and variability in the diagnosis of TB infection by the IGRAs. It is beyond the scope of these guidelines to give advice on when the screening of TB infection should be performed or when and how they should be treated. The authors refer the readers to other guidelines that provided specific guidance on that.

A full version of the document can be found online.1,2

A panel of experts comprising specialists in infectious diseases, respiratory diseases, microbiology, pediatrics and preventive medicine, and a methodologist conducted a systematic review and formulated specific recommendations following the GRADE (Grading of Recommendations of Assessment Development and Evaluations) methodology.

A summary of the recommendations and synthesis and quality of the evidence used is provided in Table 1.

The panel noted the following additional considerations:

• 1.

  The resources needed for IGRAs to be implemented are low. However, implementation in non-urban settings, where a close laboratory for rapid handling of samples is required, may be difficult. This was acknowledged to be particularly important when using the T-SPOT.TB, which should be performed within the first 24h, or in the case of the T-SPOT.TB® Extend, within 48h after incubation. Furthermore, the need for venous punctures in large collectives and requirement of a reference laboratory to handle sample batches, could make the use of IGRAs difficult for contact tracing at a community level.

• 2.

  Although there was no attempt to distinguish between the two IGRAs in the final guidelines, some panel members favored the use of the T-SPOT.TB in HIV-infected (CD4 <200) and children, based on the perception of higher sensitivity over the QTF-GIT. However, it was acknowledged that the available evidence showed no clear difference between the two tests.

• 3.

  An emphasis was placed on the importance of screening before any underlying disease becomes too advanced, because this can compromise the yield of IGRAs. This was particularly relevant for patients with HIV and for candidates for transplantation or biologic therapies. Likewise, performing the QTF-GIT when leucopenia is present (particularly in HSCT) should be avoided because it increases the likelihood of indeterminate results.

• 4.

  There was no specific recommendation on whether the TST and an IGRA should be performed simultaneously or sequentially (start with TST and follow with an IGRA in case of negativity) when both tests are recommended. The panel considers that the choice of strategy should be guided by the circumstances of a given patient and the singularities of each center.

• 5.

  Regarding the use of IGRAs in the diagnostic work-up of TB, the panel emphasizes:

  • •

    Ensure rational use of IGRAs by using them only when there is a well-founded clinical suspicion.

  • •

    A positive IGRA in patients with compatible symptoms, particularly in immunosuppressed patients and children, must prompt active investigation for active TB.

  • •

    It remains important to establish a diagnosis of TB by conventional microbiologic and/or molecular methods.

• 6.

  If a baseline test is negative (IGRAs or the TST) in HIV-infected patients with very low CD4 cell counts, it should be repeated once immunity has improved with antiretroviral therapy. This approach may be useful to detect additional positive cases.

• 7.

  In general, an IGRA is preferred when testing those who are unlikely to return for TST reading, such as patients with alcoholism, drug abuse disorders, and the homeless. In these groups, the use of IGRAs may increase test completion rates.

• 8.

  Finally, the panel emphasizes the need for health care workers to adhere to TB prevention protocols in place.

All the authors provided a conflict of interest statement. M.S. declared the following conflicts of interest: he received a fee for speaking at two conferences sponsored by Inverness Medical Iberica, S.A.U., supplier of the QuantiFERON®-TB Gold In-tube in Spain, and he is the PI of a Clinical Trial assessing QuantiFERON®-TB Gold In-tube in contact-tracing, which was supplied with blood collection tubes by Cellestis, Inc. (Carnegie, Australia). Any declared conflicts of interest were not deemed relevant to preclude involvement in the deliberations of the panel.

The panel received no funding from any for-profit companies or organizations. The Spanish Society of Respiratory Diseases and Thoracic Surgery (SEPAR) hosted the in-person meetings, while SEPAR and the Spanish Society of Infectious Diseases and Clinical Microbiology (SEIMC) funded the travel and housing costs for members to attend the meetings. Neither the SEIMC nor SEPAR have direct or indirect influence on the elaboration of the guidelines.