Abstract: A method of monitoring the level of an analyte such as glucose in a subject comprises repeated steps of extracting a quantity of the analyte from the subject into a sample, for example using reverse iontophoresis, then measuring the concentration of the analyte with a method that depletes the analyte. Whereas known methods aim to deplete the analyte fully between extraction steps, the present method allows the concentration to build up with each cycle and measures successive changes in concentration. The higher concentrations present permit more reliable measurements to be made. The sample may be primed with an initial quantity of the analyte.