Methods and devices for point of care determination of heparin concentration in blood are described. Cartridges including protamine ion sensitive electrodes (ISEs) and reference electrodes and systems for automatically determining heparin concentration in the cartridges are provided. Some systems add blood to a protamine bolus sufficient to bind all heparin, leaving excess protamine. The excess protamine concentration can be determined by measuring the initial slope of the electrode potential rate of change, and comparing the slope to known protamine concentration slope values In some cartridges, an oscillating pressure source moves the blood-protamine mixture back and forth across the protamine ISE. Some systems also use a second blood sample having the heparin removed or degraded to create a blank reference sample. Protamine ISEs can include polyurethane polymer, DNNS ionophore, and NPOE plasticizer. The polyurethane may include hard segments and soft segments, where both hard and soft segments may include cyclic and straight chain aliphatic moieties having essentially no ester or ether groups. Some hard segments may include methylene diphenyl groups. Some reference electrodes have the same polymer, plasticizer, and ionophore as the measurement electrode, but with a different concentration of ionophore.