The four possible combinations that could result are:
There is a 1 in 4 chance (25%) the baby born WILL NOT have PKU and WILL NOT be a carrier of the PKU gene.
There is a 2 in 4 chance (50%) the baby born WILL NOT have PKU and WILL BE a carrier of the PKU gene.
There is a 1 in 4 chance (25%) the baby born WILL have PKU.
The statistics apply to each pregnancy and, if there are multiple pregnancies, the statistics are independent of the previous outcome(s).
PKU is found equally in both males and females. The incidence of PKU in the USA among Caucasians is approximately 1 in 10,000 to 15,000. The incidence varies by race and geography.
If left untreated, excess phenylalanine builds up in the blood stream. The excess phenylalanine passes from the blood to the brain via the blood brain barrier. The excess phenylalanine is toxic to the brain and in most cases results in profound mental retardation.
Diagnosis
Phenylketonuria is diagnosed by a pricking the heel of a baby 24 hours after birth. Blood collection too early may result in a false negative because phenylalanine has not had adequate time to accumulate in the blood stream of the baby.
The blood is put on filter paper. Dried blood spots are punched out from the filter paper and placed on an agar plate that has a strain of bacteria called bacillus subtilis on it that will grow in presence of abnormally high amounts of phenylalanine. This method is called a bacterial inhibition assay. The amount of growth around each disk is proportional to the amount of phenylalanine in the disk.
Not all babies who originally screen positively with the Guthrie test will have PKU (false positive). If the results are positive, further confirmatory tests will be conducted as the Guthrie method is a screening test and not a diagnostic test.
There is a trend in medical field to now conduct newborn screening using Tandem Mass Spectrometry. The use of tandem mass spectrometry allows for the screening not only for PKU, but also for other treatable inborn errors of metabolism.
Phenylketonuria was the first genetic disorder with an effective treatment.
Becuase the treatment is so effective at preventing mental retardation, the test for PKU is the single most common genetic test conducted worldwide.
Treatment
While there is currently no cure for PKU, dietary treatment prevents the effects PKU can have if left untreated. Dietary treatment should be started as soon as diagnosis is confirmed. The treatment consists of limiting the amount of phenylalanine intake to exactly what is required by the body.
Because phenylalanine is an amino acid that makes up protein, protein intake must be restricted to in order to restrict phenylalanine intake. In general, in most cases foods that are high in protein must be eliminated from the diet. Examples of some foods that are high in protein and in most cases must be elminated from the diet are: all meat, poultry, fish, eggs, milk, cheese, nuts, legumes, tofu or soy based products, and flour. In addition, consumption of aspartame (Nutrasweet), an artificial sweetener, must be avoided because phenylalanine is a key ingredient. (The warning "Caution Phenylketurics: Contains Phenylalanine" that appears on things like a can of diet Coke is a warning for people with PKU.) Most fruits and vegetables are allowed, however some of the higher phenylalanine fruits and vegetables must be consumed in limited quantities. Foods modified to be low phenylalanine foods such as pasta, bread, baking mixes are available for order through specialized companies via special order.
In addition, in order to provide adequate protein and calorie intake, a medical food (commonly referred to as formula) that contains all of the amino acids found in protein, except phenylalanine, must be consumed. It is important to consult with qualified medical professionals so that a proper diet prescription can be establised.
If dietary treatment is begun early and well maintained throughout life, babies with PKU will grow to be healthy adults with PKU. (Women who have PKU need to be especially careful to maintain a strict diet prior to and during pregnancy because high levels of phenylalanine in the mother in most cases has a severe effect on the fetus.) Thanks to the advances that have been made in the diagnosis and treatment of PKU today there are adults with PKU who are doctors, lawyers, dietitians, and astronomers, among other professions.
