The present invention provides novel apparatuses and methods for isolating or recovering a subset of blood cells such as leukocytes and/or circulating tumor cells from blood samples by filtration without changing the intracellular concentration of a therapeutic agent such as an anticancer drug. Contrary to the art, the apparatuses and methods of the present invention advantageously provide cell lysates from recovered cells such as leukocytes and/or circulating tumor cells without substantial dilution of a therapeutic agent such as an anticancer drug.