The difference between serum and plasma complement activity in primary renal disease.

Low serum CH50 (s-CH50) levels were found in 16 out of 574 patients with primary renal disease. However, five of these 16 patients had normal plasma CH50 (p-CH50). The most characteristic finding in the complement profile in these sera was a marked reduction in C4 and C2 hemolytic activities with a minimal reduction in C3 and C5 activities. Sera separated at 37 degrees C showed almost normal CH50 levels, nearly equal to p-CH50, while sera prepared at room temperature and at 4 degrees C showed a remarkable decrease in their CH50 levels. Further, when the serum from one patient showing this difference was added to pooled normal human serum, the C4 activity of the mixture decreased markedly at 4 degrees C but not at 37 degrees C. It is concluded that the difference is caused by cold activation of serum complement through the classical pathway in vitro.