Philadelphia chromosome Screening

The Philadelphia chromosome is detected by genetic tests on blood or bone marrow: karyotyping to visualize the translocation, fluorescence in situ hybridization (FISH) to identify the BCR‑ABL1 fusion, and quantitative RT‑PCR to detect and monitor BCR‑ABL1 transcripts. Karyotype and FISH confirm diagnosis; PCR is most sensitive for detecting low‑level disease and monitoring treatment response.

The Philadelphia chromosome (BCR‑ABL fusion) is detected by cytogenetic karyotyping (shows t(9;22)), FISH to visualize the fusion in cells, and molecular PCR/RT‑PCR (including quantitative qPCR) to identify and quantify BCR‑ABL transcripts in peripheral blood or bone marrow. Karyotype and FISH diagnose; quantitative PCR provides sensitive detection and treatment monitoring.

The Philadelphia chromosome (t(9;22)) is classically seen in chronic myeloid leukemia (CML), present in over 90% of cases. It also occurs in a subset of acute lymphoblastic leukemia (ALL), especially adult ALL, and rarely in acute myeloid leukemia or mixed-phenotype leukemias. Its presence affects diagnosis, prognosis and targeted therapy choices.

Being Philadelphia chromosome–positive means a chromosomal translocation between chromosomes 9 and 22 creates a BCR‑ABL fusion gene. This fusion encodes an abnormal tyrosine kinase that drives uncontrolled white blood cell growth. It is most commonly found in chronic myeloid leukemia and some acute lymphoblastic leukemias, and it guides diagnosis, prognosis, and treatment with targeted tyrosine kinase inhibitors.

Diagnosing acute myeloid leukemia begins with clinical suspicion and a complete blood count plus peripheral smear to detect blasts. Definitive diagnosis requires bone marrow aspiration/biopsy showing ≥20% myeloblasts, supported by flow cytometry for immunophenotype and cytogenetic/molecular testing (eg, karyotype, FLT3/NPM1) to guide prognosis and therapy. Baseline chemistry, coagulation, and organ-function tests inform management.

The newest approved treatment for chronic myeloid leukemia is asciminib, an allosteric BCR-ABL1 inhibitor (STAMP inhibitor). It targets the myristoyl pocket of BCR-ABL, works in patients resistant to earlier TKIs—including those with the T315I mutation—and is used in chronic-phase CML after prior TKI therapy or for T315I-positive disease, offering a different mechanism when other TKIs fail.