A method and device are provided for measuring a level of a clinically relevant analyte (such as glucose) in a fluid (such as blood). The device includes a flow path for conducting said fluid through the device; a detection chamber arranged on said flow path; and detector means arranged to detect analyte levels in the fluid in said chamber, wherein: said detection chamber contains a predetermined amount of an analyte such that that analyte mixes with fluid in the detection chamber to form, at the detector means, a calibration sample of the fluid at a time after the arrival of the fluid in said detection chamber, and said detector means is arranged to detect a first analyte level of an unadulterated sample of the fluid at a first time which is before the formation of said calibration sample and to detect a second analyte level of said calibration sample at a second time which is after the formation of said calibration sample.