Microsample analysis is highly bene cial in blood-based testing where cutting-edge bioanalytical technologies enable the analysis of volumes down to a few tens of microliters. Despite the availability of analytical methods, the di culty in obtaining high-quality and standardized microsamples at the point of collection remains a major limitation of the process. Here, we detail and model a blood separation principle which exploits discrete viscosity di erences caused by blood particle sedimentation in a laminar ow. Based on this phenomenon, we developed a portable capillary-driven micro uidic device that separates blood microsamples collected from nger-pricks and delivers 2 μL of metered serum for bench-top analysis. Flow cytometric analysis demonstrated the high purity of generated microsamples. Proteomic and metabolomic analyses of the microsamples of 283 proteins and 1351 metabolite features was consistent with samples generated via a conventional centrifugation method. These results were con rmed by a clinical study scrutinising 8 blood markers in obese patients.