|Timothy L. Lim 1, Víctor R. De Jesús 1, Donald H. Chace 2, Nancy K. Meredith 1, Joanne V. Mei 1, W. Harry Hannon 1.|
1. Newborn Screening Quality Assurance Program, Centers for Disease Control and Prevention, Atlanta, GA 30341
2. The Pediatrix Center for Research and Education, Pediatrix Medical Group, 1301 Concord Terrace Sunrise, FL 33323
|Background: The use of tandem mass spectrometry (MS/MS) for the routine analysis of amino acids and acylcarnitines from dried-blood spots (DBS) has become the standard method in most newborn screening activities in the United States. The Newborn Screening Quality Assurance Program (NSQAP) has added 3-hydroxyisovalerylcarnitine (C5OH) into its quality control and proficiency testing DBS materials in 2009 to improve C5OH screening and achieve harmonization of MS/MS results across laboratories. Screening for C5OH by MS/MS is important since elevated levels of this analyte can be associated with several disorders including 3-hydroxy-3-methylglutaryl-CoA-lyase (HMG-CoA) deficiency, 3-methylcrotonyl-CoA carboxylase (3MCC) deficiency, and â-ketothiolase deficiency, among others. Consequently, setting the cutoffs has been a challenge in order to reduce false-positive results while not missing a presumptive positive case.|
Methods: NSQAP prepared DBS materials enriched with C5OH at levels near the decision point and clearly positive for classifying newborn screening specimens as abnormal/normal. In 2009, blinded PT panels prepared from C5OH specimens (9225 and 3964) were sent to participants for MS/MS analysis. Laboratories reported quantitative and qualitative (within or outside normal limits) results. Bias plots of quantitative results were constructed using reported data and results were sorted by reported analytical method (e.g., derivatized, non-derivatized).
Results: One hundred sixty seven domestic and international laboratories reported specimen 3964 as presumptive positive for C5OH in quarter 3 of 2009. The mean C5OH value and its cutoff from participants using non-derivatized method (n=29) was 2.87 µM and 1.00 µM, while participants using derivatized method (n=138) had 2.80 µM and 0.81 µM respectively. Among the labs that used the derivatized method, the mean C5OH concentration and cutoff was 2.95 µM and 0.79 µM for non-kit users, and 2.45 µM and 0.87 µM for kit users, respectively. Among the participants that used the non-derivatized method, the mean C5OH concentration and cutoff was 2.78 µM and 0.91 µM, and 3.02 µM and 1.14 µM non-kit users, respectively. The specimen 9225 had mean value of 4.77 (n=42), 5.08 (n=6) and 5.49 (n=2) using different internal standards d9-C5, d3-C8 and d3-C5, respectively.
Conclusions: Quantitative C5OH values in DBS from MS/MS varied by methods, use of kit or non-kit and use of different stable isotope internal standards. NSQAP routinely provides DBS materials to assess laboratory and method performance, guidance and surveillance to ensure continuous improvements to sustain the quality of newborn screening results worldwide.