An injectate for use in a method of measuring hematocrit of a mammalian subject, the injectate comprising: (a) an injectate carrier; and either (b) a first fluorescent marker having a first excitation wavelength and a first emission wavelength; (c) a second fluorescent marker having a second excitation wavelength and a second emission wavelength; wherein the first fluorescent marker is a dynamic molecule and the second fluorescent marker is a static molecule; or (b) a static marker having a first fluorescent tag and a second fluorescent tag, wherein the first fluorescent tag has a first excitation wavelength and a first emission wavelength, and the second fluorescent tag has a second excitation wavelength and a second emission wavelength, wherein the method of measuring hematocrit comprises: (a) calibrating the injectate to obtain a calibration identification of the injectate containing parameters of the injectate; (b) inputting the parameters of the calibration identification of the injectate to a fluorescent detector to calibrate the fluorescent detector; (c) obtaining a species specific hematocrit curve; (d) introducing the injectate into the vascular system of the mammalian subject; (e) exciting the first fluorescent marker/tag with a first excitation wavelength and exciting the second fluorescent marker/tag with a second excitation wavelength at an optical interface of an optical probe and the vascular system, or a sample taken from the vascular system; (f) measuring the first emission intensity of the first emission wavelength of the first fluorescent marker/tag and measuring the second emission intensity of the second fluorescent marker/tag at the optical interface of the optical probe and the vascular system, or a sample taken from the vascular system, using the calibrated fluorescent detector to obtain a spectrometric data set comprising a first emission fluorescent intensity curve from the first fluorescent marker/tag and a second emission fluorescent intensity curve from the second fluorescent marker/tag; (g) obtaining a ratio of the first fluorescent marker/tag to the second fluorescent marker/tag at To from the spectrometric data set to determine an apparent hematocrit of the subject from the species specific hematocrit curve; (h) obtaining a correction factor for determining the hematocrit of the mammalian subject; and (i) applying the correction factor to the apparent hematocrit to determine the hematocrit of the mammalian subject. (a) an injectate carrier; and either (b) a first fluorescent marker having a first excitation wavelength and a first emission wavelength; (c) a second fluorescent marker having a second excitation wavelength and a second emission wavelength; (b) a static marker having a first fluorescent tag and a second fluorescent tag, wherein the first fluorescent tag has a first excitation wavelength and a first emission wavelength, and the second fluorescent tag has a second excitation wavelength and a second emission wavelength, (a) calibrating the injectate to obtain a calibration identification of the injectate containing parameters of the injectate; (b) inputting the parameters of the calibration identification of the injectate to a fluorescent detector to calibrate the fluorescent detector; (c) obtaining a species specific hematocrit curve; (d) introducing the injectate into the vascular system of the mammalian subject; (e) exciting the first fluorescent marker/tag with a first excitation wavelength and exciting the second fluorescent marker/tag with a second excitation wavelength at an optical interface of an optical probe and the vascular system, or a sample taken from the vascular system; (f) measuring the first emission intensity of the first emission wavelength of the first fluorescent marker/tag and measuring the second emission intensity of the second fluorescent marker/tag at the optical interface of the optical probe and the vascular system, or a sample taken from the vascular system, using the calibrated fluorescent detector to obtain a spectrometric data set comprising a first emission fluorescent intensity curve from the first fluorescent marker/tag and a second emission fluorescent intensity curve from the second fluorescent marker/tag; (g) obtaining a ratio of the first fluorescent marker/tag to the second fluorescent marker/tag at To from the spectrometric data set to determine an apparent hematocrit of the subject from the species specific hematocrit curve; (h) obtaining a correction factor for determining the hematocrit of the mammalian subject; and (i) applying the correction factor to the apparent hematocrit to determine the hematocrit of the mammalian subject.