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Unless follow-up is possible, recommends a California pathologist, the Guthrie Bacterial Inhibition Assay should not be used to screen infants for phenylketonuria.
His recommendation is based upon a comparative study of PKU tests, reported to the annual meeting in Washington, DC, of the American Society of Clinical Pathologists. The study indicates that the Guthrie assay gives a disturbingly large number of low readings in infants with phenylalanine blood serum levels of between 4 and 7 mg/100 ml.
Since a phenylalanine level of 4 mg/100 ml or higher is usually considered a presumptive positive in screening for PKU, the Guthrie test potentially would render an unacceptably high percentage of false-negative results, said Charles W. Blumenfeld, MD, chief of the Department of Pathology, Sutter Community Hospitals and Sacramento Clinical Laboratory.
This characteristic of the Guthrie test was first noticed by Dr. Blumenfeld and co-workers, Ramona Anderson and Mary J. Wallace, in a
Comparative Study Of PKU Tests. JAMA. 1966;198(2):42. doi:10.1001/jama.1966.03110150022011