AbstractIn the finite von Neumann algebra setting, we introduce the concept of a perturbation determinant associated with a pair of self-adjoint elements H0 and H in the algebra and relate it to the concept of the de la Harpe-Skandalis homotopy invariant determinant associated with piecewise C1-paths of operators joining H0 and H. We obtain an analog of Krein's formula that relates the perturbation determinant and the spectral shift function and, based on this relation, we derive subsequently (i) the Birman-Solomyak formula for a general non-linear perturbation, (ii) a universality of a spectral averaging, and (iii) a generalization of the Dixmier-Fuglede-Kadison differentiation formula.