The invention provides for a method of determining an amount of an analyte in a blood sample (500) using a cartridge (100, 300, 600) and a blood collector (400, 800, 1200, 1513). The blood collector comprises: a mounting surface (402); a capillary structure (404) for holding the blood sample, wherein the capillary structure comprises a curved portion (406); a capillary inlet (403) for receiving the blood sample. The cartridge comprises: a receiving surface for attaching to the mounting surface; a cartridge inlet (108) for receiving the blood sample from the blood collector; a microfluidic structure (111) for processing the blood sample into a processed sample; a measurement structure (130) for enabling measurement of the processed sample to determine the amount of the analyte in the blood sample; wherein the method comprises: placing (1100, 1400) the blood sample into the capillary inlet; attaching (1104, 1404) the mounting surface to the receiving surface; rotating (1600) the cartridge about the rotational axis to transport the blood sample from the capillary structure to the cartridge inlet; rotating (1602) the cartridge about the rotational axis to transport the blood sample from the cartridge inlet into the microfluidic structure; controlling (1604) the rotation of the cartridge about the rotational axis to process the blood sample into the processed sample using the microfluidic structure; controlling (1606) the rotation of the cartridge to transfer the processed sample to the measurement structure; and measuring (1608) the amount of the analyte using the measurement structure and a measurement system.