title: Quantification of malonylcarnitine in dried blood spots by use of MS/MS varies by stable isotope internal standard composition.
abstract: BACKGROUND
  The utilization of MS/MS for the analysis of amino acids and acylcarnitines from dried blood spots (DBS) is routine in many newborn screening (NBS) laboratories. Recently, malonylcarnitine (C3DC) was shown to be elevated in the DBS of affected infants with malonic acidemia. Quantitative features were unknown, so that its measurement was an approximation. Synthesis of malonylcarnitine enabled both a study in the analytical characteristics of C3DC and a survey of its measurement in NBS laboratories.
  
  
  METHODS
  Malonylcarnitine was enriched in blood and spotted onto filter paper cards. The DBS were sent to several laboratories for analysis, and the results were returned to the Centers for Disease Control and Prevention (CDC) for evaluation. Reports included a description of the MS/MS method utilized.
  
  
  RESULTS
  A pilot proficiency survey shows a bimodal distribution of data from 98 laboratories. Analysis of proficiency data reveals the use of different stable isotope internal standards for quantification. Analysis of standard, labeled or unlabelled ((2)H(3)-octanoylcarnitine (C8), glutarylcarnitine (C5DC) and malonylcarnitine (C3DC) revealed significantly different ion detection values. Quantification in laboratories is based on the ratio of the metabolite in question to a reference stable isotope standard.
  
  
  CONCLUSIONS
  Quantification of metabolites depends upon the reference isotope standard utilized. Quantification requires describing the standards used for estimation of concentration (a pseudo-concentration) and a notation that includes a reference to the isotope standard used. This descriptive method will enable harmonization of data in screening laboratories.
authors.affiliation: Pediatrix Analytical, The Center for Research and Education, Pediatrix Medical Group, Inc., 1301 Concord Terrace, Sunrise, FL 33323, United States. donald_chace@pediatrix.com
authors.forename: Donald H
authors.initials: DH
authors.lastname: Chace
authors.forename: Timothy
authors.initials: T
authors.lastname: Lim
authors.forename: Christina R
authors.initials: CR
authors.lastname: Hansen
authors.forename: Barbara W
authors.initials: BW
authors.lastname: Adam
authors.forename: W Harry
authors.initials: WH
authors.lastname: Hannon
mesh_terms: D000138:Acidosis / Q000175:diagnosis* / Q000378:metabolism; D002331:Carnitine / Q000031:analogs & derivatives / Q000097:blood* / Q000592:standards*; D006801:Humans; D007231:Infant, Newborn; D007554:Isotopes; D008314:Malonates / Q000097:blood / Q000378:metabolism*; D012015:Reference Standards; D053719:Tandem Mass Spectrometry / Q000592:standards
chemical_list: D007554:Isotopes; D008314:Malonates; C030290:malonic acid; D002331:Carnitine