**4. Urinary proteomics**

There is a considerable interest in urine as a source of biomarkers for bladder pathologies, OAB assuming one of the foremost active areas of research. Indeed, urine is one of the most versatile biofluids as it can be easily obtained in large quantities with non-invasive methods and is stable in comparison with other fluids. Several peptides and low molecular weight proteins can be found in urine. When urine is collected for analysis, most of them should have undergone physiological proteolysis as urine may stagnate for hours in the bladder before micturition occurs. This might constitute an advantage as, in contrast with blood, serum or plasma, urinary proteins and peptides will not undergo further degradation upon collection (Kolch et al., 2005; Omenn et al., 2005).

With the advent and improvement of proteomic analysis, increasing attention has been given to the description of the urinary proteome. The number of proteins identified in the human urine is still increasing and is well above 1500 (Adachi et al., 2006; Good et al., 2010). The combination of different analysis methods and sample treatment recently allowed the identification of more than 100 000 peptides, 5000 of which were present in more than 40% of the individuals analysed (Coon et al., 2008). Several methods are currently being used to analyse the urinary proteome, including two dimensional gel electrophoresis mass spectrometry (2DE-MS), liquid chromatography MS (LC-MS), surface-enhanced laser desorption/ionization (SELDI-TOF) and capillary electrophoresis MS (CE-MS). If interested in more details and the specific advantages and disadvantages of each method, the readers are advised to seek that information elsewhere (Decramer et al., 2008).

With the recent advances in urinary proteomics, researchers are becoming more aware of problems arising during storage and preparation of samples. One important issue is the stability of urine samples. Studies show that sequential freeze/thaw cycles may affect the concentration of certain proteins. Schaub and co-workers showed that there were no significant differences in the protein profile between samples analysed before freezing and after 1 to 4 freeze-thaw cycles (Schaub et al., 2004). These results have been confirmed in subsequent studies by non-related groups (Fiedler et al., 2007; Thongboonkerd, 2007). No data is available regarding the proteomic analysis of the urine of OAB patients.
