Determination of C1q in human sera

All detectable C1q in serum is precipitated with the euglobulin fraction, and all other detectable hydroxyproline-containing protein in serum is excluded from this fraction. Since C1q contains 4.3% hydroxyproline, an estimate of its content in serum can be calculated by multiplying the hydroxyproline content of the euglobulin fraction by 23.3. The mean value of C1q in five normal male subjects determined by this method was found to be 63 microgram/ml as compared with 144 microgram/ml when determined by RID, indicating that the latter procedure seriously overestimates C1q in serum. C1q content of serum from patients with RA was found to be elevated by both procedures.