ALL cell frequencies in blood and BM samples during follow-up. (A) Frequencies of T-cell marker⁺/TdT⁺ T-ALL cells, as detected by immunofluorescence microscopy in 321 paired blood and BM samples, obtained from 26 patients.^58,60  The T-ALL cell frequencies are comparable in many pairs, but differences can occur up to 1 log. Orange, sample <3 months of follow-up; green, >3 months of follow-up. (B) (Left) Frequencies of ALL cells in 149 paired blood and BM samples from 22 T-ALL patients, analyzed by RQ-PCR of TR gene rearrangements and TAL1 deletions.⁶⁰  A strong correlation was observed between the blood and BM frequencies in T-ALL. (Right) Frequencies of ALL cells in 532 paired blood and BM samples from 62 BCP-ALL patients, analyzed by RQ-PCR of IG and TR gene rearrangements.⁶⁰  The MRD levels were significantly higher in BM compared with blood. Moreover, the ratio between the MRD levels in BM and blood was highly variable, ranging from 1 to 3 logs. Orange, sample <3 months of follow-up; green, >3 months of follow-up. (C) Frequencies of ALL cells in 141 paired BM samples (left-right) from 26 patients, showing a very high concordance.⁶²  Only in case of very low MRD levels was variation seen, mainly because of levels outside the quantitative range of the RQ-PCR assay. Orange, sample <3 months of follow-up; green, >3 months of follow-up. (D) Recovery of BM mononuclear cells after ficoll density centrifugation at different time points during follow-up in the DCOG-ALL11 protocol. Recovery of mononuclear cells is relatively low at days 33 and 78 (median, 5-8 × 10⁶). Recovery at day 78 and at later time points is much higher (median, 18-40 × 10⁶).