Karlien L M Coene1, Corrie Timmer2, Susan M I Goorden3, Amber E Ten Hoedt4, Leo A J Kluijtmans1, Mirian C H Janssen5, Alexander J M Rennings5, Hubertus C M T Prinsen6, Mirjam M C Wamelink7, George J G Ruijter8, Irene M L W Körver-Keularts9, M Rebecca Heiner-Fokkema10, Francjan J van Spronsen11, Carla E Hollak2, Frédéric M Vaz3, Annet M Bosch4, Marleen C D G Huigen1. 1. Translational Metabolic Laboratory, Department of Laboratory Medicine Radboud University Medical Centre Nijmegen The Netherlands. 2. Department Endocrinology and Metabolism Amsterdam UMC, University of Amsterdam Amsterdam The Netherlands. 3. Laboratory Genetic Metabolic Diseases, Department of Clinical Chemistry Amsterdam UMC, University of Amsterdam Amsterdam The Netherlands. 4. Department of Paediatrics, Division of Metabolic Disorders Amsterdam UMC, University of Amsterdam Amsterdam The Netherlands. 5. Department of Internal Medicine Radboud University Medical Centre Nijmegen The Netherlands. 6. Section Metabolic Diagnostics, Department of Genetics UMC Utrecht Utrecht The Netherlands. 7. Metabolic Laboratory, Department of Clinical Chemistry Amsterdam UMC, Vrije Universiteit Amsterdam Amsterdam The Netherlands. 8. Center for Lysosomal and Metabolic Diseases, Department of Clinical Genetics Erasmus MC Rotterdam The Netherlands. 9. Laboratory of Biochemical Genetics, Department of Clinical Genetics Maastricht University Medical Centre Maastricht The Netherlands. 10. Laboratory of Metabolic Diseases University of Groningen, University Medical Center Groningen Groningen The Netherlands. 11. Division of Metabolic Diseases Beatrix Children's Hospital, University Medical Centre Groningen Groningen The Netherlands.
Abstract
BACKGROUND: Reliable measurement of phenylalanine (Phe) is a prerequisite for adequate follow-up of phenylketonuria (PKU) patients. However, previous studies have raised concerns on the intercomparability of plasma and dried blood spot (DBS) Phe results. In this study, we made an inventory of differences in (pre-)analytical methodology used for Phe determination across Dutch laboratories, and compared DBS and plasma results. METHODS: Through an online questionnaire, we assessed (pre-)analytical Phe measurement procedures of seven Dutch metabolic laboratories. To investigate the difference between plasma and DBS Phe, participating laboratories received simultaneously collected plasma-DBS sets from 23 PKU patients. In parallel, 40 sample sets of DBS spotted from either venous blood or capillary fingerprick were analyzed. RESULTS: Our data show that there is no consistency on standard operating procedures for Phe measurement. The association of DBS to plasma Phe concentration exhibits substantial inter-laboratory variation, ranging from a mean difference of -15.5% to +30.6% between plasma and DBS Phe concentrations. In addition, we found a mean difference of +5.8% in Phe concentration between capillary DBS and DBS prepared from venous blood. CONCLUSIONS: The results of our study point to substantial (pre-)analytical variation in Phe measurements, implicating that bloodspot Phe results should be interpreted with caution, especially when no correction factor is applied. To minimize variation, we advocate pre-analytical standardization and analytical harmonization of Phe measurements, including consensus on application of a correction factor to adjust DBS Phe to plasma concentrations.
BACKGROUND: Reliable measurement of phenylalanine (Phe) is a prerequisite for adequate follow-up of phenylketonuria (PKU) patients. However, previous studies have raised concerns on the intercomparability of plasma and dried blood spot (DBS) Phe results. In this study, we made an inventory of differences in (pre-)analytical methodology used for Phe determination across Dutch laboratories, and compared DBS and plasma results. METHODS: Through an online questionnaire, we assessed (pre-)analytical Phe measurement procedures of seven Dutch metabolic laboratories. To investigate the difference between plasma and DBS Phe, participating laboratories received simultaneously collected plasma-DBS sets from 23 PKU patients. In parallel, 40 sample sets of DBS spotted from either venous blood or capillary fingerprick were analyzed. RESULTS: Our data show that there is no consistency on standard operating procedures for Phe measurement. The association of DBS to plasma Phe concentration exhibits substantial inter-laboratory variation, ranging from a mean difference of -15.5% to +30.6% between plasma and DBS Phe concentrations. In addition, we found a mean difference of +5.8% in Phe concentration between capillary DBS and DBS prepared from venous blood. CONCLUSIONS: The results of our study point to substantial (pre-)analytical variation in Phe measurements, implicating that bloodspot Phe results should be interpreted with caution, especially when no correction factor is applied. To minimize variation, we advocate pre-analytical standardization and analytical harmonization of Phe measurements, including consensus on application of a correction factor to adjust DBS Phe to plasma concentrations.
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