A “biomarker” (biological marker) is an indicator of a bodily function that can be objectively measured. A wide range of possible biomarkers for IBS have been considered but at present only gut transit measured using radio-isotope markers meet the criteria of reproducibility and availability. While barostat studies perform reasonably in expert centers, to do them reproducibly requires considerable effort and standardization. This makes them unsuitable for widespread use. However radio-isotope tests are expensive and of limited availability so the search for other more convenient markers including blood and stool tests is still an important goal for the future.