Occasionally, cases of PKU are missed by newborn screening. Thus, a repeat PKU test should be performed in an infant who exhibits slow development. Phenylketonuria PKU is caused by an autosomal recessive defect in the enzyme phenylalanine hydroxylase, which is required for converting phenylalanine to tyrosine.
Five percent of natural protein is composed of phenylalanine. The mutation that causes PKU is located on chromosome The specific type of mutation varies, resulting in variable severity in the clinical course of the disorder. Untreated, PKU results in severe mental retardation, but the exact pathogenesis of the mental defect is still not clear. Because treatment with a phenylalanine-restricted diet prevents the development of mental retardation, it seems likely that an increased concentration of phenylalanine in the blood is associated in some way with progressive neurologic deterioration if PKU remains untreated.
The symptoms of untreated PKU develop gradually, so they may not be noticed until irreversible mental retardation has occurred. Hence, newborn screening is essential for prevention of harmful effects. When PKU is identified in the first few weeks of life and a phenylalanine-restricted diet is instituted, intellectual development is substantially better than when the disorder is diagnosed at three to five years of age. When PKU is not diagnosed until this late age, serious damage has usually already occurred.
Initial estimates of the frequency of PKU one per 25, live births were based on institutionalized populations with profound mental retardation caused by defects that completely prevented the production of phenylalanine hydroxylase. For example, it is one per 2, live births in Turkey and one per 4, live births in Ireland.
In the United States, the frequency of PKU is one per 20, live births in California, compared with a frequency of one per 12, live births in Massachusetts.
The large population of Americans of Irish ancestry in Massachusetts may explain the increased frequency in that state. The laboratory that performs the PKU test has the responsibility of reporting the results to the physician and to the facility where the infant was born. Some laboratories also notify the parents. The blood specimen for PKU screening must be obtained at least 12 hours after birth, 9 but in recent years this has proved rather difficult because of changing obstetric practices in which patients are sometimes discharged a few hours after delivery.
The American Academy of Pediatrics recommends that a PKU screening test be repeated by two weeks of age if it was performed before the newborn was 24 hours of age. However, a second test is not necessary if the initial PKU screening is performed with the McCamon-Robins fluorometric test 8 when the newborn is between 12 and 24 hours of age. With either the McCamon-Robins test or the Guthrie test, the test should be repeated if it was performed when the newborn was less than 12 hours of age Table 1.
If Guthrie inhibition assay was used, repeat the test if the results were positive; repeat testing is not required if the results were negative. Repeat the test if the results were positive; repeat testing is not required if the results were negative. The blood phenylalanine concentration in newborns is normally 0.
Not all states, however, use 2 mg per dL as the cut-off value. Blood phenylalanine levels are generally slightly lower in breast-fed infants than in bottle-fed infants.
Breast milk contains only 12 to 14 mg of phenylalanine per ounce, compared with 24 to 28 mg per ounce in formula. Because PKU is a heterogeneous disorder, phenylalanine levels vary greatly in infants with the disease. Such cases are mild and permit production of phenylalanine hydroxylase, which converts phenylalanine to tyrosine, as in the normal person.
To be completely certain of the diagnosis, follow-up testing is recommended in male infants with initial phenylalanine levels between 3 and 10 mg per dL and in female infants with values between 4 and 10 mg per dL. Although initial PKU screening demonstrates positive results in 1 percent of infants, there is only a 10 percent chance that an infant with an initial positive result has the disorder false-positive rate of 90 percent.
False-negative results are rare. Infants should be given a normal diet until a definitive diagnosis is made. When the initial newborn screening test and the second test show positive results, a confirmatory quantitative test, such as the McCamon-Robins fluorometric test, should be performed by a laboratory at a referral metabolic center. The infant should be referred to a center capable of providing the required medical, nutritional and laboratory services for infants with PKU.
While the phenylalanine-restricted diet requires the assistance of health care professionals experienced in the management of PKU, the family physician should continue to provide routine care, including immunizations. Follow-up developmental testing before kindergarten is recommended. Testing is performed on a small amount of blood obtained on a filter paper collection device, air dried, and submitted to a newborn screening laboratory.
Only two sources of filter paper have been approved by the FDA for blood collection in the United States. These papers must meet the national criteria for acceptable performance Hannon et al. The screening system is designed to minimize false negative results, while avoiding overburdening the medical system with false positive results. The variation in practice has a potential impact on the effectiveness of newborn screening strategies in that not all families receive the same level of care.
The Committee on Genetics of the American Academy of Pediatrics published a policy statement on newborn screening in September of American Academy of Pediatrics, Included in this policy statement is some basic background material on newborn screening as well as fact sheets providing detailed information on many of the conditions that are commonly tested for in various states. The fact sheets were developed to assist pediatricians in understanding the specific characteristics and the strengths and weaknesses of the testing methods.
This policy statement acknowledges the following; "Newborn screening is an individual function of each state; therefore, screening programs are not uniform throughout the United States. Great variations in practice are evident in all areas of testing including which conditions are included in the testing protocols. All but one state in the U. Maryland mandate screening, as do four provinces in Canada as well as Puerto Rico.
Parental refusal is permitted in all but four U. Some U. Some states offer various educational services and follow-up, while others do not offer such services. For states that charge a fee, some bill the patients while others bill the referring doctor, hospital or third party provider.
Funding sources and services covered by these funding sources vary greatly. Methods for tracking and screening also vary among providers. All states in the U. A draft of the Executive Summary: Recommendations from the Newborn Screening Task Force titled, "Serving the Family from Birth to the Medical Home, Newborn Screening: A Blueprint for the Future", also addresses the great variation from state to state in practice in all components of the newborn screening process.
Department of Health and Human Services. Because of this variation in practice, the task force noted that not all newborns have access to the same level of care.
Recommendations were made to improve the newborn screening program in the U. The debate over whether a newborn screening program should be voluntary or mandatory is long standing and as yet unresolved.
This debate is tied to the issue of effectiveness of testing protocols. Along with this deliberation is the consideration of parental informed consent. Discussions surround the questions of whether parental informed consent should be required, is desirable but not really possible, is indicated in some circumstances but not others and so on. Informed consent is a major issue because American health care practices are grounded in the respect for patient autonomy.
In addition, with the exception of child abuse or neglect, parents are considered to be in the best position to make medical decisions for their children.
In the practice of medicine in America today, it is the rare case where patients and families do not have the right to participate in their health care decisions. Because of this, the determination to exclude parents in the decision process for newborn screening tests has been questioned Fost, ; ; Paul, Holtzman et al. The results of the study showed that a consent process could improve mothers' knowledge about the cause and effect of the disorders tested for, the effectiveness of treatment, the type of testing done, and the interpretation of the results.
In addition the authors state that, "Requiring parental consent for neonatal screening is cost beneficial; parental knowledge is increased without incurring significant time or effort costs, and with extremely few refusals. The study also attempted to distinguish between informing parents and obtaining consent when asking parents how they would like to be involved in such decisions. On the other hand, Statham and colleagues wrote a letter to the British Medical Journal concerning mothers consenting to newborn screening Statham et al.
Informed consent is a part of this newborn screening process. These researchers attempted to assess the knowledge base of the mothers about newborn screening after they left the hospital with their babies. They found that the knowledge base was low although the mothers believed themselves to be informed.
They conclude that these results challenge the notion that women are giving informed consent. If you would like to breastfeed, talk your health care provider. Breast milk does contain Phe, but your baby may be able to have a limited amount, supplemented by the Phe-free formula. Regardless, your child will need to stay on a special low-protein diet for life. A PKU diet usually means avoiding high-protein foods such as meat, fish, eggs, dairy, nuts, and beans.
Instead, the diet will probably include cereals, starches, fruits, a milk substitute, and other items with low or no Phe. Your child's health care provider may recommend one or more specialists and other resources to help you manage your baby's diet and keep your child healthy.
There are also a variety of resources available to teens and adults with PKU. If you have PKU, talk to your health care provider about the best ways to take care of your dietary and health needs. The information on this site should not be used as a substitute for professional medical care or advice.
Contact a health care provider if you have questions about your health. Phenylketonuria PKU Screening. What is a PKU screening test? The test is easy, with virtually no health risk.
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Learn how we develop our content. To learn more about Healthwise, visit Healthwise. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated. British Columbia Specific Information In British Columbia, prenatal and newborn screening is offered free to all families. Top of the page. Test Overview A phenylketonuria PKU test is done to check whether a newborn baby has the enzyme needed to use phenylalanine in his or her body.
Why It Is Done A phenylketonuria PKU screening test is done to see whether a newborn baby has the enzyme needed to use phenylalanine in his or her body. How To Prepare You do not need to do anything before your baby has this test. How It Is Done Your baby's heel is cleaned with alcohol, and then the heel is poked with a small needle.
How It Feels Your baby may feel a sting or a pinch with a heel stick. Risks Usually, there are no problems from a heel stick. Results A phenylketonuria PKU test is done to check whether a new baby has the enzyme needed to use phenylalanine in his or her body.
What Affects the Test Reasons the results may not be helpful include: Your baby was born early premature. A baby who weighs less than 2. Your baby has been drinking milk for less than 24 hours. Best results occur after your baby has been breastfeeding or drinking formula for 2 full days.
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