Comparison of amino acids and acylcarnitines assay methods used in newborn screening assays by tandem mass spectrometry.
BACKGROUND
The analysis of amino acids (AA) and acylcarnitines (AC) by tandem mass spectrometry (MS/MS) is performed in newborn screening laboratories worldwide. While butyl esterification assays are routine, it is possible to detect AAs and ACs as their native free acids (underivatized). The Centers for Disease Control and Prevention's Newborn Screening Quality Assurance Program provides dried blood spot (DBS) quality control (QC) and proficiency testing (PT) programs for numerous MS/MS analytes. We describe empirical differences between derivatization and non-derivatization techniques for selected AAs and ACs.


METHODS
DBS materials were prepared at levels near, above and below mean domestic laboratory cut-offs, and distributed to program participants for MS/MS analysis. Laboratories reported quantitative and qualitative results. QC DBS materials were assayed in-house following established protocols.


RESULTS
Minor differences (<15%) between quantitative values resulting from butyl esters and free acid techniques were observed for the majority of the analytes. Mass spectrometric response from underivatized dicarboxylic acid acylcarnitines was less intense than their butyl esters.


CONCLUSIONS
The use of underivatized techniques may also result in the inability to differentiate isobaric acylcarnitines. Laboratories should establish their own protocols by focusing on the decisions that identify test results requiring additional follow-up testing versus those that do not.
De Jesús
Víctor R
VR
Newborn Screening Quality Assurance Program, Centers for Disease Control and Prevention, 4770 Buford Highway, NE, Mail Stop F-19, Atlanta, GA 30341, USA. vdejesus@cdc.gov
Chace
Donald H
DH
Lim
Timothy H
TH
Mei
Joanne V
JV
Hannon
W Harry
WH
D000596:Amino Acids / Q000032:analysis* / Q000097:blood / Q000737:chemistry; D000440:Butanols / Q000737:chemistry; D002331:Carnitine / Q000031:analogs & derivatives* / Q000032:analysis / Q000097:blood / Q000737:chemistry; D006801:Humans; D007231:Infant, Newborn; D007930:Leucine / Q000032:analysis / Q000097:blood / Q000737:chemistry; D008659:Metabolic Diseases / Q000175:diagnosis; D008715:Methionine / Q000032:analysis / Q000097:blood / Q000737:chemistry; D015997:Neonatal Screening / Q000379:methods*; D010172:Palmitoylcarnitine / Q000032:analysis / Q000097:blood / Q000737:chemistry; D010649:Phenylalanine / Q000032:analysis / Q000097:blood / Q000737:chemistry; D011786:Quality Control; D053719:Tandem Mass Spectrometry / Q000379:methods*
D000596:Amino Acids; D000440:Butanols; C116917:acylcarnitine; C053168:glutarylcarnitine; C003223:propionylcarnitine; D010172:Palmitoylcarnitine; D010649:Phenylalanine; D008715:Methionine; D007930:Leucine; C008698:octanoylcarnitine; D002331:Carnitine