The devil is in the details

From Switzerland comes this study which, at first glance, is rather non plussed about the role that IGRAs can play in detecting latent tuberculosis in chronic haemodialysis patients.

From the abstract;

Conclusions. In this population, QFT was superior to TST for detecting LTBI, but both IGRAs and TST have important limitations, and are unreliable for screening for LTBI.

However the full article is particularly scathing of both TST and T Spot TB

Our results show that association between probable LTBI (defined as: chest radiography suggestive of prior infection and/or established ‘at risk’ contact with a patient with contagious TB) and results of one of the IGRA tested (QuantiFERON Gold In tube test: QFT) was highly significant. This was not the case, however, for the T-SPOT.TB or the TST.

In this study, QFT was twice as effective at detecting probable LTBI (46%, Table 1) as either the T-SPOT.TB (22%) or TST (25%). Even after adjustment for age, and previous BCG vaccination, haemodialysis patients with probable LTBI had an OR of 4.6 of having a positive QFT (Table 2). Conversely, OR for detecting latent tuberculosis for either T-SPOT.TB or TST was not significant.

Furthermore, detection of patients with prior TB was very low: among five patients with prior TB, T-SPOT.TB and TST identified only one, while QFT identified two. As previously reported, the best predictor of a positive TST was prior BCG vaccination. QFT (8%) and T-SPOT.TB (11%) had similar rates of indeterminate tests.

In a specific population when any reliable result is difficult to obtain (due to chronic illness) QFT has shown to be the most effective.

Furthermore, T Spot TB has more not less indeterminates.