FLUORESCENCE IN-SITU HYBRIDIZATION (FISH) IS A RELIABLE DIAGNOSTIC-TOOL FOR DETECTION OF THE 9-22-TRANSLOCATION

The fluorescence in situ hybridization (FISH) technique for detection of the 9;22 translocation was compared with the ''gold standard'' of c onventional cytogenetics. For this purpose, both methods were applied to 81 bone marrow aspirates and/or peripheral blood specimens comprisi ng 50 CML cases and controls from 31 patients without CML. Independent ly, core biopsies of these 81 patients were investigated by three hist opathologists. Conventional karyotype analysis from unstimulated bone marrow cells was successful in 71/81 cases and demonstrated the Ph-chr omosome in 42/46 CML patients. With FISH, results were obtained in all 81 cases investigated, confirming fusion of the bcr and abl genes in all cytogenetically Ph-positive patients. Among the five Ph-chromosome -negative specimens bcr/abl fusions were detected in only one patient. The percentage of cells found to be Ph-positive by both methods was c orrelated, but in individual cases considerable differences in the num bers of Ph-positive cells were observed. Different results may be due to selection of cells after in vitro cultivation predominantly, FISH p roved to be a very reliable technique for specimens that do not contai n dividing cells. With FISH, large numbers of cells can easily be scor ed which is an advantage compared to conventional cytogenetics. Theref ore, this method is particularly suitable for those whose therapy is b eing monitored or a relapse is suspected. However, the FISH results sh ould be evaluated critically with respect to the practical limit of se nsitivity since non-specific fusion signals can also be observed in a small percentage of cells in non-CML cases. It is suggested that each laboratory define its own threshold of bcr/abl fusion signals for diag nosing Ph-positive CML by FISH.