1. A device for evaluating a liver condition of a subject, the device comprising: one or more sensors adapted to sense an isotope level of a metabolic product of labeled methacetin or a salt or a derivative thereof in a breath of the subject; and a processor adapted to receive data indicative of said isotope levels of said metabolic product of labeled methacetin and to compute a fluctuation parameter by estimation of the physiological noise in a percentage dose recovery (PDR) curve and/or a delta over baseline (DOB) curve of an isotope labeled methacetin or a salt or a derivative thereof, in comparison to a smooth curve, wherein a value of the fluctuation parameter being at or above a predetermined threshold is indicative of at least one liver condition of the subject, and wherein a value of the fluctuation parameter being below a predetermined threshold is indicative of a normal liver condition.