A method of measuring a metabolic activity (MA) of a cell is provided. The method comprising independently measuring in an extracellular environment of the cell, time-dependent acidification profiles due to secretion of: (i) non-volatile soluble metabolic products; (ii) non-volatile soluble metabolic products and volatile soluble metabolic products; and (iii) volatile soluble metabolic products; wherein at least one of the time dependent acidification profiles is indicative of the metabolic activity of the cell. Also provided are clinical methods which make use of the assay.