Methods and devices for the measurement of clinically relevant analytes in fluids are provided. The devices and methods are particularly relevant for detection of glucose in blood and allow internal calibration of measurements. One device includes a flow path for conducting the fluid through the device; a predetermined amount of the analyte arranged on the flow path such that the analyte mixes with fluid that passes it to form a calibration sample of the fluid and detector means arranged on the flow path for detecting respective analyte levels in an unadulterated sample of the fluid and in the calibration sample. Another device provides a predetermined amount of a calibration analyte of a different species to the clinically relevant analyte arranged on the flow path, and first and second detector means for detecting the levels of the clinically relevant analyte and the calibration analyte.