MSACL 2019 EU Abstract

Comparison of Phenylketonuria Screening with a Fluorimetric Method and with Tandem Mass Spectrometry

Vanja Čuk (1), Andraž Šmon (1), Žiga Iztok Remec (1), Daša Perko (1), Blanka Ulaga (1), Adrijana Oblak (2), Ajda Biček (2), Mojca Žerjav Tanšek (3), Urh Grošelj (3), Ana Drole Torkar (3), Tadej Battelino (3), Barbka Repič Lampret (1)
(1) Unit of Special Laboratory Diagnostic, University Children's Hospital, Ljubljana, Slovenia, (2) Clinic of nuclear medicine, Ljubljana, Slovenia, (3) Department of Pediatric Endocrinology, Diabetes and Metabolic Diseases, University Children's Hospital, Ljubljana, Slovenia

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Vanja Cuk (Presenter) University Children's Hospital >> POSTER (PDF)

Presenter Bio: I am employed in Pediatric Clinic at the University Medical Centre Ljubljana, Unit for Special Laboratory Diagnostics for 10 years. 2 years ago I have completed the master's study of Laboratory biomedicine.
At work I helped with the implementation of routine expanded newborn screening in Slovenia.

Relevant Financial Disclosures (within past 24 months) No relevant financial relationship(s) to disclose. Conflict mitigation NOT REQUIRED.

Introduction: Early detection of diseases by newborn screening (NBS) is necessary for correct and timely clinical decisions. One of the early methods for NBS of phenylketonuria (PKU) was a fluorimetric method for quantification of phenylalanine (Phe). This method is still used in many countries in south-eastern Europe. The introduction of expanded NBS with tandem mass spectrometry (MS/MS) enabled screening for many diseases, including PKU. Slovenia is now in a unique position to compare the methods because it screens for PKU with a fluorimetric method detecting Phe and also using MS/MS for expanded NBS, also measuring Phe and tyrosine.
Objectives: Evaluation of MS/MS for screening of PKU and comparison with the fluorimetric method.
Methods: Fluorimetric method was performed using the Neonatal Phenylalanine kit from Perkin Elmer. The expanded NBS method is performed on a MS/MS in MRM mode, samples prepared using a nonderivatised kit NeoBase™ 2 Non-derivatized MS/MS kit from Perkin Elmer. Cut-off of Phe for both methods was 120 µmol/L, on the MS/MS we have an additional ratio Phe/Tyr for PKU screening with the cut-off 2.15.
We compared measurements of nearly 7000 newborn blood spots. The descriptive statistics, Bland-Altman analyses and Spearman correlation coefficient were calculated. The numbers of true positives and false positives for both methods were compared.
Results: Phe concentrations were 21 – 296 µmol/L, with the mean of 46 µmol/L (MS/MS method) and 10 – 280 µmol/L, with the mean of 67 µmol/L (fluorimetric method). Spearman correlation coefficient had a value of 0.49. Bland-Altman analysis comparing MS/MS method with the fluorimetric method in absolute values had a bias 21 µmol/L (SD 15.9 µmol/L), in percent difference the bias was 36 % (SD 21 %). The fluorimetric method yielded 16 positive results, one of the patients being a true positive. The MS/MS method flagged only the one PKU patient as positive.
Conclusion: The MS/MS and the fluorimetric method have a moderate correlation and the Bland-Altman analyses show that the fluorimetric method gives higher results. Nevertheless, the cut-off for MS/MS was the same as on the fluorimetric method, because the literature data and laboratory experiences do not warrant lower cut-offs. There were no false positives by MS/MS, which will occur after screening more newborns, yet it still showed that MS/MS was better suited for PKU screening. Both methods did not have any false negatives. Our results show that MS/MS is more suited for NBS of PKU and as Phe and Phe/Tyr are screened as part of the expanded NBS, MS/MS screening for PKU is also more cost and time effective.