Summary
Interpretation of protein C (PC) levels in a given individual has several limitations.
A normal PC activity does not necessarily exclude a genetic deficiency nor can a reduced
level confirm it. Measuring PC amidolytic activity in 9,648 healthy blood donors has
allowed identification of demographic factors which cause variation in PC activity
and further hinder interpretation. PC activity displays a log normal distribution
and significant variation with age. This is most marked in young adult males when
mean PC activity rises from 0.86 iu/ml (15–19 years) to 1.04 iu/ml (45–49 years; P <0.0001). Pre-menopausal females, who for most age ranges, have mean PC activity
below their male contemporaries, show a less marked rise with age until the menopause
when PC activity rises further. The use of hormonal contraceptive preparations is
associated with an increase in mean PC activity of 0.05–0.08 iu/ml while smoking habit
has no influence on PC activity.
In view of these findings we strongly recommend the use of age and sex restricted
reference ranges when interpreting PC activity.
