Methods, apparatus, and systems for filtering compacted test responses are disclosed. The methods, apparatus, and systems can be used, for example, to remove the effects of unknown test values. For instance, in one embodiment, a compacted test response from a compactor of a circuit-under-test is received. In this embodiment, the compacted test response includes one or more compacted test response values that are dependent on one or more respective unknown values. The compacted test response is filtered to remove the dependency of at least some of the compacted test response values on the one or more respective unknown values, and a filtered test response is output. Various filtering circuits and testing systems are also disclosed.