The postinduction level of minimal residual disease (MRD) was quantified with a competitive polymerase chain reaction (PCR) technique in 104 children with acute lymphoblastic leukemia (ALL) diagnosed between June 1993 and January 1998 and followed for a median of 4.2 years. A significant correlation was found between the MRD level on day 15 (D15) and day 29 (D29) after the start of induction therapy (r s ‫؍‬ 0.70, P < .0001). The 15 patients with T-cell disease had higher D29 MRD than those with B-lineage ALL (P ‫؍‬ .01). Age was positively related to D29 MRD (r s ‫؍‬ 0.32, P ‫؍‬ .001). The 16 patients who had a relapse had higher D15 and D29 MRD levels than the patients who stayed in remission (median levels D15, 1% versus 0.1%, P ‫؍‬ .03; D29, 0.4% versus 0.01%, P ‫؍‬ .0001). No patients with a MRD level less than 0.01% on D29 have so far had a relapse, whereas the 7-year probability of event-free survival for patients with higher MRD levels was 0.52 (P ‫؍‬ .0007). The group of patients with a D29 MRD less than 0.01% included patients with T-cell disease, white blood cell count more than 50 ؋ 10 9 /L at diagnosis, or age 10 years or older, and could not be identified by up-front criteria. The best-fit Cox model to predict the risk of relapse included D29 MRD (P ‫؍‬ .004) and age (P ‫؍‬ .009).

Summary We hypothesized that Wilms tumour 1 gene (WT1) expression levels in acute myeloid leukaemia (AML) patients might have predictive value and reveal molecular relapse kinetics. WT1 level was determined at diagnosis, during therapy and post‐therapy follow‐up in 89 patients who reached first complete remission (CR1) (952 samples, median 8 samples/patient, range 2–38). CR1 bone marrow (BM) WT1 level above normal (based on 39 healthy donors) was an independent adverse prognostic factor regarding both disease‐free survival [hazard ratio (HR) 4·46, P = 0·001] and overall survival (HR 2·62, P = 0·019). By grouping 34 BM and 99 peripheral blood (PB) complete remission samples in monthly intervals prior to clinical relapse, disease development was delineated and a simple mathematical model constructed, that allowed for the prediction of relapse detection rates (RDRs) as well as median times [tms] from WT1 positivity to clinical relapse. BM sampling was required to obtain RDRs above 93% and tms above 67 d. Acceptable RDRs and tms (81% and 44 d, respectively) could be acquired by bimonthly PB sampling. In conclusion, CR1 WT1 expression is an independent prognostic factor in AML. According to our model, BM is superior for relapse prediction, but PB samples are useful when shorter sampling intervals are possible.

Individualized PCR strategies hamper comparability of molecular results between different laboratories in several fields of medicine. To harmonize BCR-ABL mRNA quantification an international multicenter trial involving 37 laboratories in 14 countries was initiated using 10 samples, each containing various dilutions (10, 2, 1 and 0.1%) of b3a2 or b2a2 BCR-ABL positive in normal leukocytes and negative controls. A novel control plasmid (pME-2) was designed for external calibration containing BCR-ABL and glucuronidase-b (GUS) sequences. Median BCR-ABL/ABL ratios were 9.1, 1.8, 0.85 and 0.11% in b3a2 samples and 9.5, 1.6, 0.84 and 0.11% in b2a2 samples. Median BCR-ABL/GUS ratios were 3.4, 0.77, 0.37 and 0.042% in b3a2 samples and 2.8, 0.48, 0.29 and 0.031% in b2a2 samples. The coefficients of variation were 0.62 for ratios BCR-ABL/ABL and 1.03 for ratios BCR-ABL/GUS. Five of 37 evaluable participating laboratories (13%) detected low BCR-ABL copy numbers in negative control samples; one laboratory failed to detect BCR-ABL in a low-level sample. We conclude that the use of a common control plasmid does indeed improve comparability of BCR-ABL mRNA quantification results. However, further standardizing efforts like introducing a calibrator and regular control rounds are needed.