SLC5A7 and phenylketonuria: However, performance data from 2017 to 201826 27 indicates that 92.7% of babies with a CHT positive screening result had a clinical referral initiated within 3 working days of sample receipt by the NBS laboratory, this compared with 100% of babies with MSUD, GA1, IVA, MCADD and 99.1% of babies with PKU.