1: Pharm Res. 2007 Oct;24(10):1897-901. Epub 2007 May 12. Assessment of the "skin reservoir" of urea by confocal Raman microspectroscopy and reverse iontophoresis in vivo. Wascotte V, Caspers P, de Sterke J, Jadoul M, Guy RH, Préat V. Unité de Pharmacie Galénique, Université Catholique de Louvain, Avenue E. Mounier, UCL 7320, 1200, Brussels, Belgium. PURPOSE: To demonstrate the "skin reservoir" of urea by confocal Raman microspectroscopy in vivo and to evaluate its impact on the non-invasive monitoring of the analyte by reverse iontophoresis. METHODS: Urea was extracted iontophoretically over a 2-h period across the skin of adult volunteers and patients with chronic kidney disease. Confocal Raman microspectroscopic profiles of skin were recorded before and after 30 min of current application. RESULTS: Urea extraction was higher at the beginning of current passage, but then decreased to achieve stable values after 2 h of iontophoresis. After 30 min of iontophoresis, the Raman spectra highlighted a clear depletion of urea at the surface of the skin. Lactate distribution was also modified both at the surface and deeper into the skin. CONCLUSIONS: A source of urea in the skin, unrelated to the concentration circulating in the blood, was strongly suggested by extracted urea flux observed over time and by the Raman spectroscopy. This "urea reservoir" must be removed before systemic urea levels can be non-invasively monitored by reverse iontophoresis. PMID: 17497081 [PubMed - in process] Related Links Monitoring of urea and potassium by reverse iontophoresis in vitro. [Pharm Res. 2007] PMID:17380270 Simultaneous extraction of urea and glucose by reverse iontophoresis in vivo. [Pharm Res. 2004] PMID:15553226 In vivo confocal Raman microspectroscopy of the skin: noninvasive determination of molecular concentration profiles. [J Invest Dermatol. 2001] PMID:11231318 Reverse iontophoresis: a non-invasive technique for measuring blood urea level. [Pediatr Nephrol. 2003] PMID:12898373 Noninvasive glucose monitoring by reverse iontophoresis in vivo: application of the internal standard concept. [Clin Chem. 2004] PMID:15155544