Time domain viscometry is disclosed in the context of a thrombectomy system, wherein the aspirate composition may span saline (≈1 cP) and blood (≈4 cP) to blood/thrombus mixtures (˜10 cP to ˜10,000 cP) to a clog (>10,000 cP). An objective of a suction thrombectomy system is to aspirate low-viscosity blood at low suction levels concomitantly with aspirating high-viscosity thrombus at high suction levels. Time domain viscometry comprises measuring the time required for fluids of differing viscosities to flow into an elastically deformed conduit. One or more fluid standards (e.g., saline, blood, etc.) may be used for in-situ calibration of the viscometer. Ratiometric analysis of the resulting measurements permits time domain differential viscometry wherein the viscosity of an unknown fluid is determined with respect to a fluid of known viscosity. Viscometric thrombectomy systems are enabled to discriminate between blood and thrombus and adjust the aspiration rate accordingly.