Polymer density functional theory is used to test the accuracy of the Derjaguin approximation in describing the interaction between two spherical colloidal particles immersed in a polymer solution. The density functional equations are solved numerically in a cylindrical geometry and thus a rigorous comparison is afforded, within the density functional framework. The density functional theory provides an exact (albeit numerical) treatment for ideal polymers. We investigate both nonadsorbing and telechelic chains, as well as consider the behavior of widely polydisperse samples (living chains). We make comparisons with the commonly used penetrable sphere models for polymers and are able to identify strengths and weaknesses in the latter. We find that the Derjaguin approximation performs surprisingly well, even for particle sizes that are comparable to the radius of gyration of the polymers.