BCR/ABL FISH

Function

The BCR/ABL FISH (Fluorescence In Situ Hybridization) test is a cytogenetic analysis used to detect a specific genetic abnormality known as the Philadelphia Chromosome. This occurs when a piece of chromosome 9 (containing the ABL1 gene) breaks off and attaches to chromosome 22 (containing the BCR gene), creating the BCR-ABL1 fusion gene. This gene produces a protein that signals white blood cells to divide uncontrollably.

Why it is Ordered

This test is a critical diagnostic tool in hematology. It is ordered to:

• Confirm a diagnosis of Chronic Myeloid Leukemia (CML).
• Identify a specific subtype of Acute Lymphoblastic Leukemia (ALL).
• Monitor 'Minimal Residual Disease' (MRD) in patients undergoing treatment.
• Determine the effectiveness of Tyrosine Kinase Inhibitor (TKI) therapy.

Associated Conditions

The primary condition associated with a positive BCR/ABL FISH result is Chronic Myeloid Leukemia (CML), where the Philadelphia chromosome is present in about 95% of cases. It is also found in a subset of adult and pediatric patients with Acute Lymphoblastic Leukemia (ALL), where its presence often indicates a more aggressive disease course requiring specific targeted therapies. Because FISH can detect individual cells containing the fusion gene, it is much more sensitive than standard karyotyping. In the modern era of oncology, this test is used to track 'molecular response'—if the percentage of positive cells drops to zero, the patient is considered to be in complete cytogenetic remission.