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Laboratory Services Branch, Ontario Ministry of Health, Etobicoke, Ontario, Canada.
a Author for correspondence: Chemistry Section, Laboratory Services Branch, Ontario Ministry of Health, 81 Resources Rd., Etobicoke, Ontario, Canada M9P 3T1, or Box 9000, Terminal A, Toronto, Ontario, Canada M5W 1R5. Fax 416-235-6281.
We used ROC plots to evaluate the clinical performance of the Guthrie, Wallac, and Isolab assays for newborn phenylketonuria (PKU) screening and assessed the screening discriminatory power of these three assays by the area under the ROC plot, Youden's J index, and the likelihood ratio. The use of these plots not only allows us to pinpoint the exact cutoff value in screening, but also provides a direct comparison of these three different assays in clinical outcome performance. The optimum cutoff for the newborn PKU screening is a blood phenylalanine concentration of 0.30, 0.27, and 0.18 mmol/L for the Guthrie, Wallac, and Isolab assays, respectively. We conclude that the Wallac and Isolab kits, like the Guthrie assay, are suitable for newborn PKU screening.
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