The analysis of the complement activation product SC5 b-9 is applicable in neonates in spite of their profound C9 deficiency

Native complement factors and complement activation products were measured in healthy neonates (n = 72) and in a group of infants with premature prolonged rupture of the membranes (PPROM) without sepsis (n 10). Vitronectin concentration in normal cord blood was not correlated with gestational age, and the median value was 86.0 % of adult values. This was markedly higher than other native complement factors studied (factor B: 35.9 %, C4: 45.1 %, C3: 56.2 %). The concentration of C9 showed a positive correlation with gestational age and was very low, 10.8 % of normal adult values in cord blood and 8.3 % in the patients. Fifteen percent of the neonates had C9 levels lower than 2 % of adult values. The complement activation products Bb and SC5b-9 were significantly elevated in the patients (159 % and 130 % of control values, respectively), indicating alternative and terminal pathway activation. In contrast, C4 be and C3 he levels were not increased. The maximum amount of SC5 b-9 which could be generated in the neonatal sera by cobra venom factor was highly correlated with C9 concentration (r(s) = 0.86, p = 0.0001) The profound C9 deficiency found in neonates is correlated with gestational age, limits the capacity to form bacteriolytic C5 b-9 (m) and may predispose for severe invasive bacterial infection. The plasma level of SC5 b-9 under normal conditions was very low, only 0.3% (0.1 %-3.0 %) of the values obtained after CVF activation of the same samples. Therefore, we suggest that the analysis of SC5 b-9 is applicable also in neonates, in spite of their extremely low C9 levels.