GeneXpert Essay for BCR-ABL Compared to RT-PCR for Clinical Management of CML

Abstract 4416

Reverse transcriptase-polymerase chain reaction (RT-PCR) is considered the most sensitive method available for detecting low copy numbers of the BCR-ABL gene fusion. The difference between the BCR-ABL Ct (threshold cycle) and ABL Ct is expected to represent the ratio of the two populations of mRNAs and ultimately the percentage of neoplastic cells present.

An international scale (IS) has been proposed for BCR-ABL RQ-PCR measurements. To enable testing centers to gain access to the IS, the Adelaide laboratory initiated a process to develop and validate laboratory-specific conversion factors (CFs) that can be used to convert local values to IS values. Recently, Cepheid introduced its GeneXpert-based assay for the identification of the BCR-ABL gene fusion in cells from blood samples. This system comprises a walkaway self-contained instrument that combines cartridge- based microfluidic sample preparation with reverse transcriptase-polymerase chain reaction-based fluorescent signal detection and BCR-ABL and ABL Ct determination. The CF provided by Cepheid for this procedure is 0.47. We tested this BCR-ABL fusion detection system, compared with a classical RT-PCR analysis as a clinical diagnostic tool for CML patients.

We tested 19 patient peripheral blood samples by both methods. The negative control group included 3 blood samples, obtained from 3 patients with hematological disorders unrelated to BCR-ABL gene fusion: one with essential thrombocythemia, one with chronic myelomonocytic leukemia, one with T-cell-prolymphocytic leukemia. The remaining 16 clinical samples belonged to 12 patients with an established diagnosis of CML in chronic phase and to one patient with Philadelphia chromosome-positive acute lymphoblastic leukemia (p210).

GeneXpert software reported as negative 2 samples with a numerical threshold limit calculated on Ct (i.e. BCR-ABL was not detected at a detection limit of 0.00046%); the 3^rd negative control was indicated as BCR-ABL invalid: this result was probably due to the high proportion of ABL detected as consequence of hyperleukocytosis (WBC 559,6 × 10⁹/L) in T-Cell prolymphocytic leukemia patients. Therefore, the performance of GeneXpert test in this series was: Sensitivity 100%, Specificity 0,66%, Positive Predictive Value 100%, Negative Predictive Value 100%.

Among the series of 16 true positive samples GeneXpert revealed mean 8,67; mean standard deviation 27,55; median 1,48. Whereas in the same series RT-PCR results were: mean 37,09; mean standard deviation 59,33; median 3,37.

In conclusion GeneXpert essay for BCR-ABL could be an important tool for diagnose and monitor CML but require complete clinical data for the correct interpretation of results.