Interpretation of Results
The Guthrie assay is sensitive enough to detect serum phenylalanine levels of 180-240 μmol/L (3–4 mg/dL). In healthy normal people, phenylalanine levels are usually under 120 μmol/L.
When an elevated level of phenylalanine is detected, the laboratory notifies the infant's practitioner, who explains the result to the family and arranges testing to determine the cause of the high phenylalanine levels.
There are a number of causes of hyperphenylalaninemia, and less than 10% of the positive results obtained by the screening program are confirmed as due to phenylketonuria (PKU). There are other metabolic diseases that can produce hyperphenylalaninemia, but false positive results can also occur due to unexplained mild, transient elevations, prematurity, parenteral nutrition, or contamination of the filter paper specimen. False negative results can be produced by antibiotics in the blood sample. Misleadingly low results can also occur if a sample is taken too soon after birth, since phenylalanine levels rise steadily with age and protein feeding.
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