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Other tests may be done on blood or bone marrow samples to look for DNA changes. Fluorescent in situ hybridization (FISH) is a very sensitive lab test that uses fluorescent dyes todetect some chromosome changes that can't be seen with standard cytogenetic testing. It also provides results within a few days, which is why many centers now use this test. Polymerase chain reaction (PCR) is another very sensitive DNA test that can find some chromosome changes too small to be seen with a microscope, even if very few leukemia cells are present in a sample.