This invention describes a one-step technique for the simultaneous label-free detection and concentration of blast cells from a blood sample. Enrichment of blast cells is achieved using a closed loop microfluidics system, allowing continuous removal of waste and non-target cells to generate concentrated samples of high purity without the need for specific targeting of proteins by antibodies. The technique is highly effective for samples which cannot be purified in a single run. The application of detecting rare blast cells for monitoring minimal residual disease in leukemia patients is demonstrated. The sensitivity of the invention promotes the detection of blast cells in blood samples of early-stage patients.