Activated clotting time (ACT) tests detect blood clotting time based on the viscosity changes of a test sample, using a ferromagnetic washer lifted to the top of a test chamber and then dropped from the top via gravity; a drop time greater than a preset threshold value indicates clotting of the test sample. Blood samples which have high levels of heparin usually produce very weak clots that may easily be destroyed by the lifting movement of the washer. But if the clot threshold is set low to detect the weak clots, false detections occur during early testing cycles when activators are not fully suspended during the mixing cycle. Improved algorithms for lifting the washer and adjusting over time enable accurate detection of weak clots.