Determining Reference Ranges for Total T₄ in Dried Blood Samples for Newborn Screening.

Area of Science:

• Biochemistry
• Neonatal Medicine
• Clinical Chemistry

Background:

• Newborn screening programs rely on accurate biochemical markers.
• Total thyroxine (tT₄) is a key thyroid hormone measured in neonatal screening.
• Establishing reliable reference intervals in dried blood spots (DBSs) is essential for accurate interpretation.

Purpose of the Study:

• To define reference intervals for tT₄ in DBSs for newborn screening.
• To assess the utility of direct tT₄ measurement in DBSs for premature and term-born infants.
• To determine if gestational age-specific reference ranges are necessary.

Main Methods:

• Retrospective analysis of tT₄ concentrations in DBSs collected over 21 weeks.
• Inclusion of premature infants (<30 weeks gestational age) and term-born infants.
• Assessment of tT₄ stability in DBSs to determine appropriate sample collection timeframes.

Main Results:

• A strong correlation was observed between tT₄ levels and gestational age in premature infants.
• This correlation necessitates the development of age-specific reference ranges for premature neonates.
• For term-born infants, tT₄ ranges showed no significant variation with gestational age, supporting a single reference range.

Conclusions:

• Gestational age-specific reference intervals for tT₄ in DBSs are vital for accurate assessment of premature infants.
• A universal reference interval for tT₄ in DBSs is applicable to term-born infants.
• Direct measurement of tT₄ in DBSs can be a valuable tool in neonatal screening, with tailored reference ranges.