Rheumatoid factor in the serum of hepatitis C virus-infected patients: an increase in the titre during cold storage.

Storage of serum at 4 degrees C elevated the titre of rheumatoid factor (RF) as measured by nephelometry with IgG-coated latex in 12 of 38 specimens positive for hepatitis C virus. Increased RF following cold storage was not detected in HCV-negative sera. Most of the sera showing the cold-dependent elevation of RF had decreased complement hemolytic activity (CH50) after the cold storage. The RF titre elevated by cold storage decreased to the level of fresh serum by the addition of guinea pig serum as a complement supplement in a dose-dependent fashion. Thus, in the serum of some HCV-infected patients, the affinity of RF appears to be weak, and the binding between the RF and IgG coated on latex may be inhibited by the complement components of fresh serum.