  Inspired by recent IRIS observations, we forward model the response of the Fe
XXI 1354 \AA\ line to fundamental, standing, linear fast sausage modes (FSMs)
in flare loops. Starting with the fluid parameters for an FSM in a straight
tube with equilibrium parameters largely compatible with the IRIS measurements,
we synthesize the line profiles by incorporating the non-Equilibrium Ionization
(NEI) effect in the computation of the contribution function. We find that both
the intensity and Doppler shift oscillate at the wave period ($P$). The phase
difference between the two differs from the expected value ($90^\circ$) only
slightly because NEI plays only a marginal role in determining the ionic
fraction of Fe XXI in the examined dense loop. The Doppler width modulations,
however, posses an asymmetry in the first and second halves of a wave period,
leading to a secondary periodicity at $P/2$ in addition to the primary one at
$P$. This behavior results from the competition between the broadening due to
bulk flow and that due to temperature variations, with the latter being
stronger but not overwhelmingly so. These expected signatures, with the
exception of the Doppler width, are largely consistent with the IRIS
measurements, thereby corroborating the reported detection of a fundamental
FSM. The forward modeled signatures are useful for identifying fundamental FSMs
in flare loops from measurements of the Fe \MyRoman{21} 1354 \AA\ line with
instruments similar to IRIS, even though a much higher cadence is required for
the expected behavior in the Doppler widths to be detected.
