Comparison of FISH and quantitative RT-PCR for the diagnosis and follow-up of BCR-ABL-positive leukemias

Background: For Philadelphia chromosome positive (Ph+) leukemias (chronic myelogenous leukemia [CML], acute lymphoblastic leukemia [ALL], and rare other leukemias), both allogeneic transplantation and treatment with tyrosine kinase inhibitors offer chances of molecular remission (the molecular marker being consistently undetectable). Molecular remission is defined as a reduction in the quantification of BCR-ABL transcripts to an undetectable level by molecular diagnostic methods, and is considered as a surrogate marker for cure or long-term disease control. The molecular diagnostic methods including fluorescence in situ hybridization (FISH) and quantitative reverse transcription-polymerase chain reaction (QRT-PCR) are more sensitive than classical cytogenetic analysis for the detection of BCR-ABL positive cells. QRT-PCR, due to its superior sensitivity, is considered the gold standard for the follow-up of Ph+ leukemias treated with imatinib. Aim: The objective of our study was to compare the diagnostic and clinical usefulness of FISH and QRT-PCR at different timepoints for Ph+ leukemias. Patients and methods: We investigated 23 unselected patients with Ph+ CML (n = 21) or Ph+ ALL (n = 2) at 77 different timepoints in a comparative study with both FISH and QRT-PCR using commercially available reagents in a routine laboratory. Results: Our study demonstrated a good correlation of QRT-PCR with FISH in detecting the BCR-ABL fusion gene among patients with CML or ALL (coefficient of correlation = 0.77493, p < 0.0001, using Spearman's correlation procedure). All newly diagnosed or untreated cases were positive with both methods. Lower coefficients of correlation were found when FISH and QRT-PCR were correlated with the white blood cell count (WBC). An overall concordance of FISH and QRT-PCR (being either negative or positive in both tests) was found in 65 cases (84.4%) and a discrepancy identified in 12 cases (15.6%). Conclusions: We confirm that QRT-PCR allows precise measurement of low levels of BCR-ABL transcripts and can serve as a sensitive indicator for minimal residual disease. In addition, we demonstrate in most cases a good correlation of QRT-PCR with FISH in detecting the BCR-ABL fusion gene among patients with CML or Ph+ ALL. FISH is not suitable for monitoring minimal residual disease.