Definition of Pediatric Age Groups

The rapid physical maturation that occurs between birth and adulthood is well known. Logically, it would be anticipated that these changes would result in altered responses to

Adapted from Michele Danish

St. Joseph Health Services, Providence, Rhode Island Mary Kathryn Kottke

Cubist Pharmaceuticals, Inc., Lexington, Massachusetts

Table 1 Classification by Age of the Pediatric Population

Age group

FDA classification

ICH classification


Preterm newborn infants

Neonate or term newborn infant

Infant and toddlers



Conception to birth

Birth to 1 mo

2 yr to onset of puberty Onset of puberty to adult

<37 wk of gestational age 0-27 day 28 day-23 mo 2-11 yr

12-16 or 18 yr (depending on regions)

xenobiotics. Within the first five years of life 95% of children have been prescribed medications. Yet only few new molecular entities that have potential usefulness in pediatric patients have pediatric labeling at the time of approval.

In its continuing effort to improve the safety and efficacy of drugs in the pediatric population, the U.S. Food and Drug Administration (FDA) has defined five subgroups of this population based on age. Each subgroup is not homogeneous but does contain similar characteristics that are considered milestones in drug safety development. The International Conference of Harmonization (ICH) approach uses similar age groups in relation to developmental stages in children (ICH Topic Ell Clinical Investigation of Medicinal Products in the Paediatric Population). The FDA and ICH classifications are listed in Table 1.

Age classifications do not provide an all-inclusive method for establishing pediatric doses because it assumes that the body weight is appropriate for the child's age and do not take into account developmental differences, disease state, etc. On the basis of current knowledge, the most accurate pediatric doses are determined utilizing both age and weight. Most formulary will normalize children doses by body weight; occasionally body surface area (calculated from height and weight in square meter) is used as it correlates better with many physiological phenomena. It should be reserved for potent drugs as it limits the risk of overdosing. Apart from very young infants who show dramatic age-related differences in drug disposition, allometric scaling can also be used. In this dosing system, clearance and volume of distribution are scaled to body weight, which relates morphology and physiological function to body size (2).

The classification listed in Table 1 will be utilized throughout this chapter. Caution must be exercised when referring to observations in the neonatal period, however, since premature and/or very low birth weight neonates often have drug responses and disposition significantly different from those of full-term neonates. These differences will be clearly identified in the text.

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