Heavy on the down strokes, light on the up strokes...the pen is mightier than the sword

The latest NICE IGRA guidelines, in draft form, are available here.

It is a very disappointing document; despite acknowledging the failings of the Mantoux skin test

Diagnosis has in the past been reliant on the TST but this has poor specificity if there has been BCG vaccination or environmental mycobacterial exposure, which can lead to false positive results.

and the advantages of IGRA

Interferon-gamma tests showed little evidence of being affected by prior BCG vaccination, and showed stronger correlation with exposure categories than did TST. This was shown in low prevalence groups, in household contacts, and in outbreak situations. The specificity of interferon-gamma tests seemed better, and there was less potential for false positive results.

they are unable to see their way clear

In the absence of a gold-standard reference test, it is not possible to measure directly the sensitivity and specificity of a new test for latent tuberculosis.

In general they advocate "offering" the Mantoux first however

Those with positive results (or in whom Mantoux testing may be less reliable) should then be considered for interferon-gamma immunological testing

By their own definition the Mantoux is always less reliable