A parent brings a 2-week-old infant to a clinic for a phenylketonuria rescreening blood test. The test indicates a serum phenylalanine level of 1 mg/dL (60.5 mcmol/L). The nurse reviews this result and makes which interpretation?
A.
It is inconclusive
B.
it is negative
C.
it requires rescreening at age 6 weeks.
D.
It is positive
The Correct answer and Explanation is:
The correct answer is B. it is negative.
Explanation
Phenylketonuria (PKU) is a genetic disorder that results in an inability to metabolize phenylalanine, an amino acid found in many protein-containing foods. Early detection through newborn screening is crucial because high levels of phenylalanine can lead to severe neurological damage if not managed with a strict diet.
In this scenario, the infant’s serum phenylalanine level is reported as 1 mg/dL (60.5 µmol/L). To interpret this result correctly, it is essential to know the standard cut-off values for phenylalanine in newborns. Typically, a phenylalanine level below 2 mg/dL (120 µmol/L) is considered normal and indicative of a negative screening result for PKU.
Here, the result of 1 mg/dL is well within the normal range, thus indicating that the infant does not have PKU. If the level were above this threshold, particularly if it were above 4 mg/dL (240 µmol/L), it would raise suspicion for PKU and necessitate further testing and dietary interventions.
Option A (it is inconclusive) is incorrect because the value provided is clear and within the normal range. Option C (it requires rescreening at age 6 weeks) is also incorrect; standard practice typically does not require rescreening if the initial test result is negative. Option D (it is positive) is incorrect since a positive test would indicate a significantly elevated phenylalanine level.
In conclusion, the 1 mg/dL phenylalanine level indicates that the screening is negative for PKU, and the infant is not at risk for the condition based on this test result. Regular monitoring and appropriate dietary management can be initiated if necessary, but in this case, no immediate action is required based on the screening result.