  We use the operator product expansion (OPE) to show that non-perturbative QCD
corrections to $\Delta\rho$ can be calculated using unsubtracted dispersion
relations for either the transverse or the longitudinal vacuum polarization
functions. Recent calculations of the non-perturbative contribution to
$\Delta\rho$ based on a non-relativistic calculation of corrections to the
$\ttbar$ threshold are inconsistent with this result.
