In this study, which is the largest reported comparison of metaphase cytogenetics and MDS FISH panel testing to date, we identified abnormal FISH findings in 94 (21.7%) of 433 cases, including 6 (14%) of 43 cases with fewer than 20 available metaphase cells (suboptimal growth) and 10 (19%) of 54 cases with no growth. In contrast, in cases with a normal result in 20 or more metaphase cells, FISH studies were positive in only 6 (2.7%) of 222 cases. Of these 6 cases, 1 involved a patient with relapsed myeloma and suspected MDS in which FISH showed +5 and +8. Given that +5 is common in myeloma20 and rare in MDS, these abnormalities are likely representative of plasma cells rather than myeloid elements. The 5 remaining cases with a normal karyotype and abnormal FISH results each showed 10% or fewer abnormal nuclei. Because, by definition, numeric (gain) or structural abnormalities must be identified in at least 2 metaphase cells to be considered clonal by cytogenetics, these small clones would