title: Assessment of the "skin reservoir" of urea by confocal Raman microspectroscopy and reverse iontophoresis in vivo.
abstract: OBJECTIVE
  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.
authors.affiliation: Unité de Pharmacie Galénique, Université Catholique de Louvain, Avenue E. Mounier, UCL 7320, 1200, Brussels, Belgium.
authors.forename: Valentine
authors.initials: V
authors.lastname: Wascotte
authors.forename: Peter
authors.initials: P
authors.lastname: Caspers
authors.forename: Johanna
authors.initials: J
authors.lastname: de Sterke
authors.forename: Michel
authors.initials: M
authors.lastname: Jadoul
authors.forename: Richard H
authors.initials: RH
authors.lastname: Guy
authors.forename: Véronique
authors.initials: V
authors.lastname: Préat
mesh_terms: D000328:Adult; D006801:Humans; D007478:Iontophoresis / Q000379:methods*; D019344:Lactic Acid / Q000378:metabolism; D018613:Microscopy, Confocal; D051436:Renal Insufficiency, Chronic / Q000097:blood / Q000378:metabolism*; D012720:Severity of Illness Index; D012867:Skin / Q000378:metabolism*; D012869:Skin Absorption; D013059:Spectrum Analysis, Raman; D013997:Time Factors; D014508:Urea / Q000097:blood / Q000378:metabolism*
chemical_list: D019344:Lactic Acid; D014508:Urea