Abstract: Methods and apparatus provide filtration for concentrating analytes, such as bacteria or exosomes, of a biological sample, such as blood or urine. The technology may employ membrane devices that implement one or more tangential flow filtration processes such as in stages. An example membrane device may typically include a membrane having sides and ends. The membrane may selectively permit constituent(s) of the sample to pass through while retaining other constituents at one side. An input chamber of the device may include an inlet near one end and an outlet near the other end, and that may permit a tangential flow of the sample along the first side surface, and a trans-membrane passing of constituent(s). An output chamber of the device may be configured at the second side surface to receive the passing constituents. Such devices may be provided in a kit to facilitate targeting of a desired biological analyte concentration.