Prevalence and persistence of C1q binding activity in healthy subjects.

Samples of serum from 885 normal healthy blood donors were tested for the presence of soluble immune complex-like material by a solid-phase C1q binding assay. The majority of donors (93%) had low or undetectable levels of C1q binding activity in their sera, but 6% had levels that were clearly outside the normal distribution. When these individuals were retested after several weeks half of them still had elevated levels of C1q binding activity.