A system and method for determining a coagulation time, e.g., thrombin time, PT, aPTT, and ACT, of a blood sample deposited in a test cartridge is disclosed. The test cartridge includes a blood receptacle that is open to the atmosphere into which a blood sample is to be deposited, a vacuum port that is open to atmosphere, and a spiral capillary within the test cartridge having a capillary length and cross-section area, a first capillary end of the spiral capillary open to the blood receptacle and a second capillary end of the spiral capillary open to the vacuum port, whereby the spiral capillary is closed to atmosphere. When a blood sample is deposited in the blood receptacle, a vacuum is drawn through the vacuum port and the blood is drawn through the spiral capillary until coagulation occurs. A pressure change is detected, and the coagulation time is measured.