The real deal - an update

Forrest has linked to the updated Diel paper on the predictive value of QFT, which reinforces the failure of TST;

QFT+ = 198 out of 954
TST+ = 604 out of 954 (+5mm lump
TST+ = 242 out of 954 (-10mm lump)

and for those that progressed to active (n = 19)

QFT+ = 19 (100%)
TST 5mm+ = 17 (89.5%)
TST 10mm+ = 10 (52.6%)

To put that into words

This study found a 2-year rate of progression to active TB of nearly 15% for those who were QFT-positive, compared with 2.3% for those who were TST-positive

Obviously a 5mm TST is closer to the mark than a 10mm lump however TST false positives are ~200% of QFT+ and false negatives were significant.

Two of the subjects who progressed to disease were QFT-positive, but TST negative, each with a 0-mm induration.

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In conclusion, our results demonstrate the benefits of using the highly specific QFT assay in place of the TST in populations at risk and with a high pre-test probability of MTB infection. QFT yielded a higher positive predictive value, not only for determination of LTBI status, but, more importantly, for identifying those most likely to develop active TB disease in the near future. Moreover, whereas in clinical settings the usefulness of IGRAs fall short of that of a ‘rule-out’ test for active TB, given a diagnostic sensitivity of only 81% to 88% [2], in healthy subjects with intact immune function the sensitivity of the tests for detecting LTBI is likely much higher. This is supported by our finding that QFT had a 100% negative predictive value for progression to active TB in our study of a large body of close contacts with a high pre-test likelihood of infection.