  There is a growing interest in the automated analysis of chest X-Ray (CXR) as
a sensitive and inexpensive means of screening susceptible populations for
pulmonary tuberculosis. In this work we evaluate the latest version of CAD4TB,
a commercial software platform designed for this purpose. Version 6 of CAD4TB
was released in 2018 and is here tested on a fully independent dataset of 5565
CXR images with GeneXpert (Xpert) sputum test results available (854 Xpert
positive subjects). A subset of 500 subjects (50% Xpert positive) was reviewed
and annotated by 5 expert observers independently to obtain a radiological
reference standard. The latest version of CAD4TB is found to outperform all
previous versions in terms of area under receiver operating curve (ROC) with
respect to both Xpert and radiological reference standards. Improvements with
respect to Xpert are most apparent at high sensitivity levels with a
specificity of 76% obtained at a fixed 90% sensitivity. When compared with the
radiological reference standard, CAD4TB v6 also outperformed previous versions
by a considerable margin and achieved 98% specificity at the 90% sensitivity
setting. No substantial difference was found between the performance of CAD4TB
v6 and any of the various expert observers against the Xpert reference
standard. A cost and efficiency analysis on this dataset demonstrates that in a
standard clinical situation, operating at 90% sensitivity, users of CAD4TB v6
can process 132 subjects per day at n average cost per screen of \$5.95 per
subject, while users of version 3 process only 85 subjects per day at a cost of
\$8.38 per subject. At all tested operating points version 6 is shown to be
more efficient and cost effective than any other version.
