The present invention relates to a device for determination of the hemoglobin amount of a patient comprising: a closed gas volume (100)； a gas outlet comprising a mouth piece (47), wherein the mouth piece (47) is configured to enable inhalation and exhalation of gas into and out of the closed gas volume (100)； a means for carbon monoxide supply into the closed gas volume (100)； characterized in that the means for carbon monoxide supply comprises a carbon monoxide reservoir (15) which is provided with sensors for the determination of temperature (19) and pressure (17) of the carbon monoxide in the carbon monoxide reservoir (15).
