  We look at the question posed by Parker et al. about the effect of UV
regularisation on the power spectrum for inflation. Focusing on the slow-roll
$k$-inflation, we show that up to second order in the Hubble and sound flow
parameters, the adiabatic regularisation of such model leads to no difference
in the power spectrum apart from certain cases that violate near scale
invariant power spectra. Furthermore, extending to non-minimal $k$-inflation,
we establish the equivalence of the subtraction terms in the adiabatic
regularisation of the power spectrum in Jordan and Einstein frames.
