Accurate measurement of free carnitine in dried blood spots by isotope-dilution electrospray tandem mass spectrometry without butylation.
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.
Schulze
Andreas
A
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
Schmidt
Christiane
C
Kohlmüller
Dirk
D
Hoffmann
Georg F
GF
Mayatepek
Ertan
E
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*
D002331:Carnitine