Total
complement 4 (C4) levels, when analysed on the Beckman Array nephelometer, were found to increase number of serum specimens [predominantly from patients with hepatitis C virus (HCV)
infection] after overnight storage at 4 degrees C. In order to investigate whether the phenomenon of in vitro cold-dependent activation of
complement (
CDAC) was the explanation for this increase, paired specimens were collected from 63 patients with HCV
infection in tubes with no
anticoagulant (serum) and in tubes containing
EDTA (which inhibits complement activation). C4 levels increased after overnight storage at 4 degrees C in 33 serum specimens (52%). In contrast, no increase in C4 levels was observed in any of the 63
EDTA specimens. Immunofixation of intact and activated C4 products confirmed that complement activation had taken place in the serum specimens in which C4 levels had increased after storage. There was a higher frequency of
hepatitis C viraemia (P<0.0001), HCV antibody positivity (P<0.05) and the presence of
rheumatoid factor (P<0.05) in the group of patients in whose serum samples
CDAC had occurred (n = 33) than in the other group (
n = 30). As a result of our findings on C4 analysis in stored serum specimens, we would recommend
potassium EDTA plasma as the specimen of choice for
complement analysis on the Beckman Array.