Testing mixed-signal circuits is a challenging task requiring high amounts of human and technical resources. To overcome these drawbacks, indirect testing methods have been adopted as an efficient solution to perform specification based tests using easy to measure CUT information. In this work, the acceptance/rejection regions are encoded using octrees with arbitrary precision regardless of the complexity of the boundary separating these regions. The strategy relies on two phases: (1) A statistical training phase generates the circuit samples to encode the acceptance/rejection regions using octrees. (2) The testing phase corresponds to the actual production testing of the fabricated ICs. The first phase is performed only once and requires sufficient samples of each pass/fail region to generate the octree under realistic CUT variations. The second phase is fast and only requires to evaluate the octree using the measures of the tested IC. In order to illustrate the proposal, the method has been applied to test a band-pass Biquad filter. Successful simulation results are reported showing considerable advantages in terms of test application time.