Method and device for determining the concentration of heparin in a sample of fluid

A method for determining the concentration of heparin in a fluid sample comprising: (a) providing a fluid sample containing heparin; (b) adding to the fluid sample a solution of a dye to form a mixture of sample and dye, wherein the dye interacts with the heparin in the sample so that the absorption spectrum of the mixture of sample and dye in the visible range varies as a result of the interaction in a manner quantitatively dependent on the heparin concentration; (c) determining the absorption spectrum in the visible range of the mixture of sample and dye; and (d) calculating a spectral parameter representative of both the reduction in the absorption of the free dye in solution and the increase in the absorption of the dye-heparin complex, the value of the spectral parameter being indicative of the concentration of heparin present in the fluid sample, in order to determine the concentration of heparin present in the fluid sample.