title: Accurate measurement of free carnitine in dried blood spots by isotope-dilution electrospray tandem mass spectrometry without butylation.
abstract: BACKGROUND
  To test the feasibility of free carnitine (FC) determination in dried blood spot specimens (DBS) by stable isotope-dilution electrospray-ionisation tandem mass spectrometry (MS/MS).
  
  
  METHODS
  The MS/MS method established for newborn screening, measuring acylcarnitines by positive precursor ion scan of m/z 85 in DBS, was adapted by omitting the butylation and heating step during sample preparation. FC measurement in DBS by this non-butylating MS/MS assay was compared with the butylating MS/MS method and the spectrophotometric Cobas method.
  
  
  RESULTS
  FC measurement by the non-butylating MS/MS method meets the demands for a bioanalytical microassay with respect to linearity, detection limit (LOD), accuracy, and precision. Formation of FC was 0-1% and 1-4% in liquid samples and in DBS by the non-butylating MS/MS method, while 3-10% and 8-16% by the butylating method, respectively. Acid-catalysed hydrolysis (butanolysis) in liquid samples was higher for short-chain acylcarnitines (acetyl- and propionylcarnitine). Hydrolysis in DBS was more pronounced for long-chain acylcarnitines. FC concentrations in healthy newborns without butylation were 35% lower than those measured by the established newborn screening assay.
  
  
  CONCLUSIONS
  The non-butylating MS/MS assay provides a simple and accurate method for FC determination in DBS and represents a trivial but important adaptation of a method already used in many laboratories.
authors.affiliation: Division of Metabolic and Endocrine Diseases, Department of General Pediatrics, University Children's Hospital, Im Neuenheimer Feld 153, D-69120 Heidelberg, Germany. andreas_schulze@med.uni-heidelberg.de
authors.forename: Andreas
authors.initials: A
authors.lastname: Schulze
authors.forename: Christiane
authors.initials: C
authors.lastname: Schmidt
authors.forename: Dirk
authors.initials: D
authors.lastname: Kohlmüller
authors.forename: Georg F
authors.initials: GF
authors.lastname: Hoffmann
authors.forename: Ertan
authors.initials: E
authors.lastname: Mayatepek
mesh_terms: D002138:Calibration; D002331:Carnitine / Q000097:blood*; D006801:Humans; D006868:Hydrolysis; D007231:Infant, Newborn; D012016:Reference Values; D012044:Regression Analysis; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
chemical_list: D002331:Carnitine