Letter: C3 in cystic fibrosis.

We believe that serum-levels of individual complement components in children cannot be 
meaningfully interpreted unless they are compared with normal values obtained from 
age-matched controls. In a study of 163 healthy infants and children 
we found a statistically significant correlation of C3 levels with age, 
but not with sex or race, and we also found a 
wide range of values within any given age-group. In contrast to 
previous reports, we did not find raised C3 levels in the 
majority of our C.F. patients when the values were compared with 
projected group mean values for the appropriate age, and we did 
not find a significant elevation of the mean C3 level for 
the C.F. patients in any of the age-groups tested. 