A method of determining an amount of an analyte in a blood sample using a cartridge (100, 300, 600) and a blood collector (400, 800, 1200, 1513), wherein the cartridge is operable for being spun around a rotational axis, wherein the blood collector comprises: - a mounting surface; - a capillary structure for holding the blood sample, wherein the capillary structure comprises a curved portion, wherein the capillary structure forms at least part of a visual indicator for indicating when the capillary structure is filled with the blood sample; - a capillary inlet for receiving the blood sample; wherein the cartridge comprises: - a receiving surface for attaching to the mounting surface; - a cartridge inlet for receiving the blood sample from the blood collector, wherein the blood collector is configured such that when the mounting surface is attached to the receiving surface the capillary inlet is in fluidic connection with the cartridge inlet; - a microfluidic structure for processing the blood sample into a processed sample, wherein the microfluidic structure is fluidically connected to the inlet; - a measurement structure 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 the cartridge about the rotational axis to transport the blood sample from the capillary structure to the cartridge inlet; - rotating the cartridge about the rotational axis to transport the blood sample from the cartridge inlet into the microfluidic structure; - controlling the rotation of the cartridge about the rotational axis to process the blood sample into the processed sample using the microfluidic structure; - controlling the rotation of the cartridge to transfer the processed sample to the measurement structure; and - measuring the amount of the analyte using the measurement structure and a measurement system. - a mounting surface; - a capillary structure for holding the blood sample, wherein the capillary structure comprises a curved portion, wherein the capillary structure forms at least part of a visual indicator for indicating when the capillary structure is filled with the blood sample; - a capillary inlet for receiving the blood sample; - a receiving surface for attaching to the mounting surface; - a cartridge inlet for receiving the blood sample from the blood collector, wherein the blood collector is configured such that when the mounting surface is attached to the receiving surface the capillary inlet is in fluidic connection with the cartridge inlet; - a microfluidic structure for processing the blood sample into a processed sample, wherein the microfluidic structure is fluidically connected to the inlet; - a measurement structure for enabling measurement of the processed sample to determine the amount of the analyte in the blood sample; - placing (1100, 1400) the blood sample into the capillary inlet; - attaching (1104, 1404) the mounting surface to the receiving surface; - rotating the cartridge about the rotational axis to transport the blood sample from the capillary structure to the cartridge inlet; - rotating the cartridge about the rotational axis to transport the blood sample from the cartridge inlet into the microfluidic structure; - controlling the rotation of the cartridge about the rotational axis to process the blood sample into the processed sample using the microfluidic structure; - controlling the rotation of the cartridge to transfer the processed sample to the measurement structure; and - measuring the amount of the analyte using the measurement structure and a measurement system.