As liver diseases are a major health problem and especially the metabolic liver diseases like non-alcoholic fatty liver disease (NAFLD) rising, the demand for non-invasive tests is growing to replace liver Non-invasive tests such as carbon-labelled breath tests can provide a contribution to the evaluation of metabolic liver function. This review critically appraise the value of the 13 C-labelled microsomal breath the evaluation of metabolic liver function, and to discuss the role of P450 enzymes in the metabolism of the different probe drugs, especially aminopyrine. Although a number of different probe drugs have been used tests, the perfect drug to assess the functional metabolic capacity of has not been found. Data suggest that both the 13 C2 -aminopyrine and C-methacetin breath test can play a role in assessing the capacity of microsomal liver function and may be useful in the follow-up of patients chronic liver diseases. Furthermore, CYP2C19 seems to be an important the N-demethylation of aminopyrine, and polymorphisms in this gene may breath test values, which should be kept in mind when performing the 13 -aminopyrine breath test in clinical practice.