In acute lymphoblastic leukaemia (ALL), investigation of minimal residual disease by conventional morphology and immunology fails to detect levels of residual disease of <1 leukaemic in 10–100 normal cells. The use of polymerase chain reaction (PCR) to exploit the diversity of the complementarity determining region (CDR) and immunoglobulin variable heavy chain (VH) family specific usage has greatly improved the sensitivity up to one leukaemic cell in 105–106 normal bone marrow cells. Here we report on a prospective study of 14 patients with ALL of B-cell lineage by using a combined PCR approach which estimates levels of disease between 1:103 and 1:105. The sequential use of allele-specific oligoprimer (ASO) independent tests (using framework 1, FR1 and 3, FR3 primers with a JH consensus primer, sensivity up to 1:5×103) and ASO-dependent PCR (sensitivity up to 1:105) assays were applied to 64 bone marrow (BM) follow-up samples in a sequential array of tests.
Results presented in this study indicate high concordance of MRD among different tests for samples with level of residual disease >1:5×103. Consequently, samples positive by the FR1 and FR3 fingerprinting tests were confirmed by the more sensitive ASO-dependent tests, as expected. However, the ASO-dependent assays revealed levels of disease undetected by the FR1 and FR3 test. Although a higher level of sensitivity is provided by the ASO-dependent tests, the FR1 and FR3 fingerprinting tests allow MRD investigation in patients with oligoclonal B cell proliferations, CDR3 region of size <15bp or with ASO primers unsuitable for PCR investigation on technical grounds (i.e. background signal). If a sequential order of investigation from less (e.g. FR1 and FR3 fingerprinting) to more sensitive tests (ASO-dependent) is applied, an indirect estimate of MRD is obtained for patients with level of disease <1:103