Isospin transport ratio is a powerful method to estimate the neutron-proton (n-p) equilibration in heavy-ion collisions, and extensively used to obtain information on the asy-stiffness of the nuclear equation of state. In fact such a ratio is expected to bypass any perturbations introducing a linear transformation of the chosen observable. In particular, it is supposed to overcome contributions due to emission, either of dynamical or statistical nature, from the primary fragments formed during the collisions. In this paper we explore the validity of this assumption, looking at the quasiprojectile n-p ratio (N/Z) in peripheral and semiperipheral events for Ca+Ca reactions at 35MeV/nucleon, simulated via the antisymmetrized molecular dynamics transport model, coupled to different statistical decay codes. The statistical de-excitation of the primary fragments introduces a linear transformation at relatively high excitation energies (above 2MeV/nucleon) when the residue approaches the evaporation attractor line, while some effect is produced at lower excitation energies due to the occurrence of some nonlinearities. As for fast emissions after the end of the projectile-target interaction it is shown that they introduce a nonlinear transformation too.