A blood analyzer for performing a blood analysis on a blood sample, comprising: a removable cartridge, the removable cartridge having a flow channel adapted to receive a blood sample from a user; an instrument adapted to receive the removable cartridge, the instrument having a user interface; a light source positioned on a first side of the flow channel and configured to provide an incoming light beam to an interface of a wall of the flow channel and a sample fluid in the flow channel when the cartridge is received in the instrument; the instrument and/or cartridge having one or more on-board reagents, the one or more on-board reagents collectively being sufficient to support the desired blood analysis of the blood sample; and the instrument and cartridge together adapted to perform the desired blood analysis of the blood sample without requiring any user intervention once the removable cartridge receives the blood sample and the removable cartridge is received by the instrument and the analysis is started, except for interacting with the user interface of the instrument; the blood analyzer being characterized in that it further comprises: a detector positioned on the first side of the flow channel being configured to detect light reflected from the interface when the cartridge is received in the instrument; wherein a reflected signature of light measured by the detector is indicative of a correct or an incorrect sample fluid; and a controller configured to not provide a result and/or to issue an error message or error code to the user when the reflected signature of light measured by the detector is different from a desired reflected signature of light. a removable cartridge, the removable cartridge having a flow channel adapted to receive a blood sample from a user; an instrument adapted to receive the removable cartridge, the instrument having a user interface; a light source positioned on a first side of the flow channel and configured to provide an incoming light beam to an interface of a wall of the flow channel and a sample fluid in the flow channel when the cartridge is received in the instrument; the instrument and/or cartridge having one or more on-board reagents, the one or more on-board reagents collectively being sufficient to support the desired blood analysis of the blood sample; and the instrument and cartridge together adapted to perform the desired blood analysis of the blood sample without requiring any user intervention once the removable cartridge receives the blood sample and the removable cartridge is received by the instrument and the analysis is started, except for interacting with the user interface of the instrument; the blood analyzer being characterized in that it further comprises: a detector positioned on the first side of the flow channel being configured to detect light reflected from the interface when the cartridge is received in the instrument; wherein a reflected signature of light measured by the detector is indicative of a correct or an incorrect sample fluid; and a controller configured to not provide a result and/or to issue an error message or error code to the user when the reflected signature of light measured by the detector is different from a desired reflected signature of light. a detector positioned on the first side of the flow channel being configured to detect light reflected from the interface when the cartridge is received in the instrument; wherein a reflected signature of light measured by the detector is indicative of a correct or an incorrect sample fluid; and a controller configured to not provide a result and/or to issue an error message or error code to the user when the reflected signature of light measured by the detector is different from a desired reflected signature of light.