Approximate Consistency Condition from Running Spectral Index in   Slow-Roll Inflationary Models
  Density perturbations generated from inflation almost always have a spectral index n_s which runs (varies with the wavelength). We explore a running spectral index scenario in which the scalar spectral index runs from blue (n_s >1) on large length scales to red (n_s<1) on short length scales. Specifically, we look for a correlation between the length scale at which n_s-1=0 and the length scale at which tensor to scalar ratio T/S reaches a minimum for single field slow roll inflationary models. By computing the distribution of length scale ratios, we conclude that there indeed is a new approximate consistency condition that is characteristic of running spectral index scenarios that run from blue to red. Specifically, with strong running, we expect 96% of the slow roll models to have the two length scales to be within a factor of 2, with the length scale at which the tensor to scalar ratio reaching a minimum longer than the wavelength at which n_s-1=0. 
Daniel J.H. Chung and Antonio Enea Romano