  The details of reheating energy scale $\rho_{reh}$ is largely uncertain
today, independent of inflation models. This would induce uncertainty in
predicting primordial spectrum. Such uncertainty could be very large,
especially for spectra with large running $n_S$. We find that for some
inflation models with a large $d\ln n_S(k)/d\ln k$, $\rho_{reh}$ could be
highly restricted by current CMB observations.
