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.