Multivariate discrimination for phenylketonuria (PKU) and non-PKU hyperphenylalaninemia after analysis of newborns' dried blood-spot specimens for six amino acids by ion-exchange chromatography.

Ion-exchange HPLC was developed for testing dried blood-spot specimens from newborns. The method is suitable for quantitative confirmatory testing of abnormal specimens detected in the New York State Newborn Screening Program. Positive specimens were initially identified among all New York State newborns with semiquantitative bacterial inhibition assays (BIA) for aminoacidopathies, including phenylketonuria (PKU) and non-PKU hyperphenylalaninemia (HP), maple syrup urine disease, and homocystinuria. A selection of 1346 specimens from routine BIA screening, including 131 newborns with PKU or persistent HP, were tested by HPLC. Of 179 BIA results that were falsely positive, 98 (55%) were also falsely positive by HPLC in which the Phe/Tyr ratio was the discriminator and the threshold was set to attain 100% sensitivity. Investigation of three multivariate discriminatory methods revealed that linear discriminant analysis excluded all but 35 (20%) of the BIA false-positives.