The potential benefits of SPME for global metabolomics studies of blood/plasma in combination with LC-MS have already been demon- strated by Vuckovic et al. [5]. The results have shown that it is possible to extract hundreds of metabolites with a single coated fiber with sensitivity and precision comparable to,if not better than,that of traditional methods, such as ultrafiltration and protein precipitation. This study also demonstrated that in vivo sampling allows detection of short- lived metabolites, such as β-NAD, AMP or glutathione, not detected by any other methodology. This result clearly indicated that blood with- drawal could have a significant impact on the representativeness of the metabolome at the time of sampling.