title: Relationship between plasma and salivary melatonin and cortisol investigated by LC-MS/MS.
abstract: BACKGROUND
  Disturbance of the circadian rhythm has been associated with disease states, such as metabolic disorders, depression and cancer. Quantification of the circadian markers such as melatonin and cortisol critically depend on reliable and reproducible analytical methods. Previously, melatonin and cortisol were primarily analyzed separately, mainly using immunoassays.
  
  
  METHODS
  Here we describe the validation and application of a high-throughput liquid chromatography in combination with mass spectrometry (LC-MS/MS) method for the combined analysis of melatonin and cortisol in plasma and saliva. The LC-MS/MS method was validated according to international validation guidelines. We used this method to analyze total plasma, free plasma (as obtained by equilibrium dialysis) and saliva melatonin and cortisol in healthy adults.
  
  
  RESULTS
  Validation results for plasma and saliva melatonin and cortisol were well within the international validation criteria. We observed no difference between saliva collected by passive drooling or Salivette. Moreover, we noted a significant difference in saliva vs. free plasma melatonin. We observed on average 36% (95% CI: 4%-60%) higher salivary melatonin levels in comparison to free plasma melatonin, suggestive of local production of melatonin in the salivary glands.
  
  
  CONCLUSIONS
  The novel outcome of this study is probably due to the high precision of our LC-MS/MS assay. These outcomes illustrate the added value of accurate and sensitive mass spectrometry based methods for the quantification of neuroendocrine biomarkers.
authors.affiliation: .
authors.forename: Martijn
authors.initials: M
authors.lastname: van Faassen
authors.affiliation: .
authors.forename: Rainer
authors.initials: R
authors.lastname: Bischoff
authors.affiliation: .
authors.forename: Ido P
authors.initials: IP
authors.lastname: Kema
mesh_terms: D002853:Chromatography, Liquid; D006801:Humans; D006854:Hydrocortisone / Q000097:blood*; D008550:Melatonin / Q000097:blood*; D012463:Saliva / Q000737:chemistry*; D053719:Tandem Mass Spectrometry
chemical_list: D008550:Melatonin; D006854:Hydrocortisone
keywords: cortisol; dialysis; liquid chromatography in combination with mass spectrometry (LC-MS/MS); melatonin; plasma; saliva