Relative fu ratio measuring method based on dynamic analysis

The present invention addresses the problem of providing a method for accurately determining the unbound fraction (fu) of a compound including a high protein binding rate compound in a short time. The present invention pertains to a method for determining the relative unbound fraction ratio (relative fu ratio) of a substance to be measured between different biological samples, the method comprising: (1) a step for preparing a chamber system (I) in which adjacent chambers are separated from each other by a semipermeable membrane through which the substance to be measured can permeate; (2) a step in which a donor solution containing the first biological sample (A) and the substance to be measured is added to one chamber (donor-side chamber) in the chamber system (I); (3) a step for adding an acceptor solution containing a second biological sample (B) to a chamber (acceptor-side chamber) different from the donor-side chamber in the chamber system (I); (4) a step for measuring the concentration of the substance t