Phenylketonuria is a rare inherited metabolic disorder in which one’s inability to metabolize the amino acid phenylalanine can cause severe intellectual disability, seizures, and other health issues if left untreated. Newborn screening is necessary for early detection and timely intervention. This article considers the science behind detecting PKU in newborns and describes in detail the pattern of methodology, biochemical principles, and clinical significance.
What is Phenylketonuria?
Phenylketonuria is a rare genetic disorder resulting from mutations in the PAH gene coding for the enzyme phenylalanine hydroxylase, which catalyzes the conversion of an amino acid, Phenylalanine into Tyrosine (important for proper brain development). The deficiency or absence of the enzyme PAH leads to accumulation of Phenylalanine, and reduction in the Tyrosine synthesis, resulting in toxicity through the buildup of phenylalanine in the blood. Together with another newborn screening test, it is also important to bring an early diagnosis of PKU at an asymptomatic stage, thereby preventing irreversible damage.
How Does Newborn Screening Detect PKU?
There are several screening methods, ranging from basic to advanced, used to detect PKU after collecting a heel prick sample between 48-72 hours after birth. Blood droplets are placed on specialized filter paper and stored as dried blood spots (DBS) for further laboratory testing.
Screening Tests/Methods
1. Basic/Cost-effective (Core/Routine Tests)
- Bacterial Inhibition Assay (BIA):
Historically, the Guthrie bacterial inhibition assay was considered the gold standard. This semi-quantitative test uses the bacterium Bacillus subtilis grown in a medium with added phenylalanine. If bacteria grow around the blood spot, it indicates high phenylalanine levels in the blood.
- Fluorescence-based Biochemical/Enzymatic Immuno Assay:
Currently, the most commonly used, simple, and cost-effective method for screening PKU is a biochemical assay. This test measures the concentration of phenylalanine in dried blood samples, and high levels of phenylalanine suggest the presence of PKU.
2. Advanced Methodology (Expanded Test)
Tandem Mass Spectrometry (MS/MS):
Tandem Mass Spectrometry (TMS) is a highly sensitive method that measures the concentration of phenylalanine and its ratio to tyrosine. PKU is identified by an increase in phenylalanine levels and a change in the phenylalanine-to-tyrosine ratio. This method is more precise than the biochemical assay and can screen for several other conditions at the same time.
Diagnostics/Confirmatory Testing:
Once the screening test shows a positive result, there are a few additional methods that can help confirm the diagnosis of PKU. These are highly accurate techniques.
- Plasma or Serum Amino Acid Analysis: The positive screening results can be verified using a serum/plasma amino acid analyzer, which uses chromatography techniques.
- Next Generation Sequencing (NGS): Genetic testing can also be performed to identify specific mutations in the PAH gene. This helps to better understand the severity of the disease and allows for more effective management and treatment.
Clinical Significance:
Timely detection of PKU in newborn screening allows for prompt dietary management. In such a restricted regimen, the administration of a phenylalanine-free medical formula maintains the safe blood levels of the amino acid. This monitoring is carried out regularly to prevent neurological complications and ensure normal growth and development.
Newborn screening for PKU is essentially a preventive test. It brings both types of screening-biochemical and genetic testing together so that interventions may be introduced early, thus greatly improving outcomes for affected babies.