Implications of simultaneous tests for tuberculin and non-tuberculous mycobacteria antigen in the elderly

It is well established that old people living in institutions are more susceptible to tuberculous reactivation and exogenic infection than the general population. Therefore, annual screening with a tuberculin (T) skin test is suggested. Unfortunately, the specificity and sensitivity of this test is limited because of cross-reactivity with non-tuberculous mycobacteria (NTM) and anergy. We therefore investigated the possibility that a simultaneous test with NTM antigen might be helpful in the interpretation of the T test in the elderly. We tested 220 inhabitants of a geriatric institution in Israel simultaneously with T and Mycobacterium scrofulaceum sensitin (S). Ninety-six subjects reacted to the T test, and 64 of them reacted to S as well. In this last group, 38 reacted more to T, 22 more to S, and 4 reacted equally to both antigens. Of the 124 subjects who did not react to T, 24 did react to S and 123 were retested with T. Twenty-two of the latter group had a positive booster effect, but 9 of the positive booster responders initially reacted to S. The difference between the proportions of the positive booster effect in the responders and the non-responders to S was significant. We conclude that in an area where infection with NTM is present a simultaneous S test may clarify the meanings of initial and booster-positive responses to T in the elderly, as well as exclude anergy when the T test is negative.