The invention relates to a device for determining the hemoglobin count of a patient, having: a closed gas volume (100)； a gas outlet having a mouthpiece (47), wherein the mouthpiece (47) is designed to enable an inhalation and exhalation of gas into and out of the closed gas volume (100)； a means for supplying carbon monoxide into the closed gas volume (100)； characterized in that the means for supplying carbon monoxide has a carbon monoxide reservoir (15), which has sensors for determining the temperature (19) and pressure (17) of the carbon monoxide in the carbon monoxide reservoir (15).
