in the physical interpretation of this relation. Some have interpreted the fact that the $L^{\prime}($HCN$)$–SFR relation is more linear and has less scatter than the $L^{\prime}($CO$)$–SFR relation as evidence of a threshold density for star formation [@Wu2005; @Heiderman2010; @Lada2012]. However, the difference between these two relations can also be explained by whether the median gas density is above or below the critical density of CO (1–0) or HCN (1–0) [@Krumholz2007], and the subsequent relation between the gas density and line luminosity given the emission from sub-thermal gas [@Narayanan2008]. Furthermore, @Stephens2016 find that the kpc-scale observations of integrated galaxy properties cannot be explained by a simple summation of clumps, and suggest that the low scatter in the $L^{\prime}($HCN$)$–SFR relation is due to a universal dense gas star formation efficiency, universal stellar IMF, and universal core/clump mass functions, with the kpc scale being sufficient to sample the full mass functions as well as various evolutionary states. Thus, we do not necessarily expect the HCN traced gas to trace gas where collapse to stars is inevitable, and such a threshold is likely much higher than $10^4$ cm$^{-3}$ [@Krumholz2007b]. Nevertheless, despite potential nuances in the interpretation of the linear SFR-HCN relations, the HCN/CO ratio 