Age-dependent effect on the level of factor IX.

ABSTRACT

Levels of factor IXC and factor IXAg were measured in 120 healthy subjects with an age range of 1 to 67 years. Levels of factor IXC were lowest in prepubertal subjects, then reached a plateau in early adult life with a secondary increase in later adult life (> 45 years). Changes in factor IXAg showed a similar trend. Factor IXAg disproportionately increased in early adult life, however, with a less pronounced increase in later life, resulting in peaking of the ratio in early adult life. It is suggested that caution be exercised in interpreting low normal factor IXC levels in prepubertal children, and that interpretation of ratios of antigenic to coagulant activity may need to take subject age into consideration.