A source of error in phenylketonuria screening

The escalating number of blood specimens from late-fed premature or very sick newborns greatly increases the risk of missing the diagnosis of phenylketonuria (PKU). Babies receiving antibiotics have uninterpretable 'clear-zone' results with the traditional Guthrie bacteriologic inhibition assay (BIA). For the past year the authors have reexamined the blood phenylalanine level on specimens giving the 'clear-zone' effect by BIA by use of the McCaman-Robins chemical-fluorescent assay (CFA). Spuriously high blood phenylalanine levels occurred in four babies who were receiving ampicillin and whose specimens were collected on filter paper and autoclaved in preparation for the BIA. None of the babies proved to have PKU. The fluorescent interference caused by ampicillin resulted from the heat of autoclaving the specimen. The authors recommend that the blood specimen should not be autoclaved before analysis by either BIA or CFA.