Apparatus for use in performing impedance measurements on a subject, wherein the apparatus includes a processing system for, at each of three frequencies, determining first and second parameter values for first and second impedance parameters relating to the impedance of at least one body segment of the subject, solving simultaneous equations representing a circle defined with respect to the first and second impedance parameters to thereby determine circle parameter values, the equations being solved using the first and second parameter values at each of the three frequencies, using the circle parameter values to determine a third impedance parameter value at a respective frequency and using the third impedance parameter value to determine an indicator indicative of relative fluid levels within the body segment of the subject.
