Obtaining a phenylalanine cuttof for samples taken between 24 and 36 hours of life

CASTILLO C; SANTAPA O; CHIESA A; TORRES F,; MARINO S; ARANDA C; MACCALLINI CG; TOMMASI F; DRATLER G

CABA

Congreso; XI Congreso Latinoamericano de errores congénitos del metabolismo y pesquisa neonatal; 2019

INTRODUCTION: Determining a cut-off value is a crucialstage when starting the newborn screening, since the numberof false positives must be reduced without compromisingsensitivity. The cut offs are usually established at 36-72 hoursof life, however is increasing the number of samples takenbefore that range, resulting in patient recall.OBJECTIVE: To calculate a new cut-off value forPhenylalanine (Phe) and Phenylalanine/Tyrosine ratio(Phe/Tyr), in a cohort of patients whose samples were takenbetween 24 and 36 hours of life and to compare it with thecurrent one established at 36 -72 hours of life.MATERIALS AND METHODS: We analyzed 638 driedblood samples from full term newborns with breastmilkfeeding, obtained between 24 and 36 hours after birth. Thequantification of Phe was performed in an API3200 tandemmass spectrometer (ABSciex) using non-derivatizedMassCheck reagent (Chromsystems).The mean, the standard deviation and the 99.9 percentiles ofthe results (Phe and Phe/Tyr) were calculated. There werecompared with those published in Region 4 Stork (R4S). Thecut-off value was considered clinically validated if it wasbetween the P99 of the normal population and the P5 of thetrue positive cases for Phe reported in the R4S and between theP25 and P75 of the distribution of cut-off values for Phereported in the R4S.RESULTS: Phe: Mean = 50.1 uM, SD = 9.46, P99.9 = 91Phe/Tyr: Mean = 0.4, SD=0.19, P99.99= 2.15. Validation:Phe: Cut off = 91 uM, P99-P5=97-233, P25-75=113-150;Phe/Tyr: Cut off =2.08, P99-P5= n-3.4, P25-75=1.6-2.5;Current cut off values: Phe=115.30 uM; Phe/Tyr=1.77JIEMS Supplement with SLEIMPN 2019 Abstracts 113CONCLUSION: The cut off values obtained are differentfrom those calculated in the cohort whose sample was takenbetween 36 and 72 hours of life.When analyzing the results, we observed that the clinicalvalidation criteria are not met for Phenylalanine; reason couldbe differences in the hours of life of the samples from thedatabase.These new cut off values could be used for samples takenbetween 24 and 36 hours of life, avoiding recall these patients