Test Background FISH involves the application of fluorescent DNA probes specific to genes or genetic regions of interest that highlight abnormalities involving these regions.
In CML the pathogenic chromosome rearrangement is the t(9;22) or its variants, which gives rise to the BCR-ABL1 gene fusion. Conventional cytogenetic analysis is carried out routinely on diagnostic CML samples, in addition to a FISH using BCR/ABL1 probes for the rapid identification of the BCR-ABL1 gene fusion. Further FISH may also be used at the discretion of the laboratory.
In cases of CML, FISH is useful in the following scenarios:
• Rapid identification of BCR-ABL1 at diagnosis or sudden relapse
• Identifying and monitoring BCR-ABL1 aberrations that are cryptic at the level of conventional cytogenetics
• In cases where a sample has failed to produce metaphases, or in which the disease cells are suspected of having failed to divide.
• Cases in which only a BM/PB smear is available.
• CML patients in advanced phase often acquire additional chromosomal abnormalities in their BCR-ABL1-positive clone. FISH can be useful in clarifying these aberrations, and monitoring their presence following further treatment.
FISH probes commonly applied to CML samples include, but are not limited to, the following:
MECOM (EVI1) 3q26
Chromosome 8 centromere
Reference Range N/A
Sample Required See notes section
Sample Volume Samples would not be rejected on the basis of small volume, however, 5 mL is ideal.
Turnaround Time See notes section
Bone marrow in cytogenetic transport medium (preferred) or lithium heparin is usually the sample of choice, but peripheral blood in lithium heparin may be suitable if there are circulating blasts and/or a high white blood cell count. EDTA samples are only suitable in cases requiring FISH only (eg blood samples taken at diagnosis for exclusion of rapid identification of BCR-ABL1 when a bone marrow sample will shortly follow). If in doubt please use lithium heparin. Samples which are non-sterile, clotted or collected in sodium citrate, fixative or saline are not suitable.
To ensure appropriate analysis and interpretation it is important to provide clear and concise clinical information.
• Diagnosis. Rapid FISH tests (identification of BCR-ABL1 at diagnosis): 95% should be reported within 3 working days. The results of conventional cytogenetic analysis will be included in a final report, including any additional FISH, within 10 calendar days if rapid FISH was negative, or within 21 calendar days if rapid FISH was positive.
• Follow-up. Post treatment follow-up samples are treated as routine; 95% should be reported within 21 calendar days according to national guidelines. If sudden relapse and/or progression are suspected and a more rapid result is required please inform the laboratory.