Erythrocyte-incorporated 6-mercaptopurine metabolite levels are not affected by recent drug administration during maintenance therapy for childhood acute lymphoblastic leukemia

•Maintenance therapy in childhood ALL prevents leukemic relapse.•Poor adherence to maintenance therapy explains 40–50% of ALL relapses.•Ery-TGN and ery-MeMP metabolites can be used for adherence monitoring.•Ery-MeMP and ery-TGN levels are not influenced by an individual 6-MP dose.•Blood for metabolites analysis can be drawn irrespective of timing of 6-MP intake.