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Power supply noise (PSN) has become a critical issue during high-quality at-speed testing. Discrepancies between the circuit's switching activity during functional and test mode can cause overtesting and lead to yield loss. Alternatively, reduced PSN effects around critical paths can result in undertesting the chip, causing test escapes. To achieve a high-quality at-speed test, it is necessary to solve these problems simultaneously. Our previous work introduced a noise index model (NIM), which can be used to predict the mismatch between expected and real path delays. This paper quantitatively investigates and compares NIM values for critical paths during functional and test mode. We then propose a test pattern modification method that harnesses the NIM. The method fills a subset of the don't care bits in partially specified test vectors such that the worst observed functional NIM for the targeted critical path is replicated during test mode.