Regression analyses showed that individuals with high M
tuberculosis exposure were more likely to have a positive IGRA.
This is consistent with a recent household contact study in
Nigeria; children exposed to a smear-positive index case were
more likely to be Quantiferon-in-tube positive than those with
a smear-negative index case.30 Our data suggest that the use of
an exposure gradient to M tuberculosis in high burden settings
such as South Africa is feasible. Exposure gradients need to be
further validated and refined in tuberculosis endemic settings to
elucidate differences between risk factors for household and
other sources of transmission. Our composite contact score
allowed for multiple factors including smear status, duration
and proximity of exposure. The enhanced performance of the T-SPOT.TB compared with the QTF as assessed through
exploratory ROC analysis in relation to M tuberculosis exposure
also requires further validation.