Peripheral blood (PB) measurement of Dara sensitivity. (A) Representative example of matched bone marrow (BM) and PB samples from a patient stained for MM cells analyzed by flow cytometry, showing a similar but smaller population in the blood. (B) Overall, the frequency of MM cells in the blood was nearly 1 order of magnitude less than that observed in the BM. (C) Similar to the BM My-DST, the PB sample showed that MM cells were more sensitive to Dara if the patient had been off that drug for >1 year. (D) The CD38 expression levels on MM cells in the PB were highly congruent with those in the BM from matched populations. Pearson r value is shown with its associated P value; the solid line represents linear regression. (E) CD38 levels on circulating MM cells in the blood are low <12 months from the last dose of Dara, then increase after >12 months. Statistical comparisons shown were performed using unpaired t tests.