In-system deterministic tests are used in safetysensitive designs to assure high test coverage, short test time, and low data volume, typically through an input-streaming-only approach that allows a quick test delivery. The output side of the same scheme is, however, inherently vulnerable to unknown (X) states whose sources vary from uninitialized memory elements to the last-minute timing violations. Typically, X values degrade test results and thus test response compaction requires some form of protection. This paper presents two X-masking schemes that complement the primary (or level-A) blocking of unknown values by filtering out those X states that escape the first stage of masking and shall not reach a test response compactor or test result sticky-bits deployed by the on-chip compare framework. Experimental results obtained for eleven industrial designs show feasibility and efficiency of the proposed schemes altogether with actual impact of X-masking on various test-related statistics.