Phenylalanine Heel Prick

Pregnancy

Report in 72Hrs

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No Fasting Required

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Newborn screening for PKU.

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Phenylalanine Heel Prick - Comprehensive Medical Test Guide

Why is it done?
- Screens newborns for phenylketonuria (PKU), an inherited metabolic disorder that causes deficiency of the enzyme phenylalanine hydroxylase
- Detects elevated phenylalanine levels in blood that could lead to severe intellectual disability and neurological damage if untreated
- Part of mandatory newborn screening programs in most developed countries, performed within 24-48 hours of birth
- Enables early diagnosis and intervention through dietary management before neurological complications develop
- Identifies patients with milder forms of phenylalanine metabolism disorders, including atypical PKU and hyperphenylalaninemia
Normal Range
- Normal Reference Range: Less than 2.0 mg/dL (or less than 120 μmol/L) of blood phenylalanine
- Borderline/Elevated: 2.0-4.0 mg/dL (120-240 μmol/L) may require repeat testing and further investigation
- Positive/Abnormal: Greater than 4.0 mg/dL (greater than 240 μmol/L) indicates suspected PKU or related disorder
- Units of Measurement: mg/dL (milligrams per deciliter) or μmol/L (micromoles per liter)
- Interpretation: Normal results indicate the infant does not have classical PKU or significant phenylalanine metabolism disorder. Abnormal results suggest possible PKU, atypical PKU, or persistent hyperphenylalaninemia requiring confirmatory testing
Interpretation
- Normal Results (<2.0 mg/dL): Indicates normal phenylalanine metabolism; infant is not affected by PKU and can consume standard infant formula and breast milk without dietary restrictions
- Elevated Results (>4.0 mg/dL): Suggests classical PKU or atypical PKU; immediate referral to metabolic specialist and repeat testing required to confirm diagnosis
- Borderline Results (2.0-4.0 mg/dL): Inconclusive and may indicate benign hyperphenylalaninemia or transient elevation; repeat heel prick typically performed at 7-14 days of life to differentiate
- Classical PKU Diagnosis: Phenylalanine levels persistently >20 mg/dL; typically severe neurological consequences without early dietary intervention
- Atypical PKU: Results between 10-20 mg/dL; may be due to defects in cofactor regeneration (tetrahydrofolate deficiency); requires specialized testing and metabolic counseling
- Factors Affecting Results: Timing of collection (must be after protein feeding established), prematurity, low birth weight, liver disease, infection, or certain medications may influence results
Associated Organs
- Primary Organ Systems: Brain (central nervous system), liver (metabolic processing), kidneys (phenylalanine excretion)
- Phenylketonuria (PKU): Autosomal recessive inherited disorder affecting the enzyme phenylalanine hydroxylase; leads to accumulation of phenylalanine and its metabolites in blood and tissues
- Hyperphenylalaninemia: Conditions resulting in elevated blood phenylalanine including benign persistent hyperphenylalaninemia and defects in tetrahydrofolate metabolism
- Neurological Complications if Untreated: Severe intellectual disability, seizures, behavioral problems, developmental delay, cortical atrophy, white matter abnormalities
- Other Associated Manifestations: Musty/mousy body odor, light skin pigmentation, eczema, behavioral problems, attention deficits, impulsivity; maternal PKU can cause fetal damage during pregnancy
- Related Metabolic Disorders: Deficiency of dihydrofolate reductase, deficiency of GTP cyclohydrolase I, and other cofactor defects affecting phenylalanine metabolism
Follow-up Tests
- Confirmatory Venous Phenylalanine Test: If heel prick results are abnormal, venous blood sample is collected for precise quantitative phenylalanine measurement and confirmation
- Plasma Amino Acid Profile: Comprehensive analysis of all amino acids including tyrosine ratios to differentiate classical PKU from other hyperphenylalaninemia forms
- Urine Organic Acids: Detects phenylalanine metabolites (phenylpyruvate, phenyllactate) and helps characterize the type of phenylalanine metabolism disorder
- Phenylalanine Loading Test: Oral phenylalanine challenge to assess metabolic capacity and confirm benign hyperphenylalaninemia diagnosis
- Pterins Testing: Urine and blood pterin analysis to identify defects in tetrahydrofolate synthesis and cofactor metabolism
- Genetic Testing: DNA sequencing of PAH gene (and cofactor metabolism genes if applicable) to identify specific mutations and confirm PKU diagnosis
- Repeat Heel Prick: Borderline results warrant repeat testing at 7-14 days of life to distinguish transient from persistent elevation
- Neuroimaging (MRI/CT): Brain imaging may be performed to assess for white matter changes or other structural abnormalities in untreated PKU cases
- Ongoing Monitoring: Confirmed PKU cases require periodic blood phenylalanine level monitoring (typically weekly initially, then monthly) to assess dietary management effectiveness
Fasting Required?
- Fasting Requirement: NO - Fasting is not required for the phenylalanine heel prick test
- Timing Consideration: Test should be performed after infant has established protein feeding (24-48 hours after birth) to allow adequate phenylalanine accumulation for accurate detection
- Feeding Recommendation: Infant should have received at least 24 hours of milk feeds (breast milk or formula) to establish phenylalanine levels before collection
- Special Instructions: No medications need to be withheld; sample collection requires only minor heel prick; infant should be warm and comfortable during collection for optimal blood flow
- Sample Collection: Blood is collected via heel prick onto specially prepared filter paper cards (Guthrie cards); requires only 50-100 microliters of blood
- Transportation: Filter paper cards must be air dried and transported to the laboratory within specified timeframe (typically 3-5 days) at room temperature
- Parent/Guardian Preparation: Newborn should be cleaned gently before collection; parents should be informed of test purpose and normal result timeframe (typically 1-2 weeks)

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