  Kinetic treatments of drift-tearing modes that match an inner resonant layer
solution to an external MHD region solution, characterised by
$\Delta^{\prime}$, fail to properly match the ideal MHD boundary condition on
the parallel electric field, $E_{\parallel}.$ In this paper we demonstrate how
consideration of ion sound and ion Landau damping effects achieves this and
place the theory on a firm footing. As a consequence, these effects contribute
quite significantly to the critical value of $\Delta^{\prime}$ for instability
of drift-tearing modes and play a key role in determining the minimum value for
this threshold.
