Background Circulating immune complex (CIC) are forming in a normal immune response, as a consequence of attachment of specifically reactive antibodies to antigens. As a result, most antigens are neutralised and/or eliminated. However, in some circumstances, CIC deposits in tissues with subsequent phlogogenic responces.
Systemic lupus erythematosus (SLE), systemic sclerosis (SSc) and rheumatoid arthritis (RA) are organ nonspecific autoimmune diseases with a high prevalence of positive test for CIC. It seems that systemic character of these diseases is due to vascular entrapment of CIC in various tissues.
Application of various methods on the same serum sample regularly gives a different level of CIC.
Objectives Our objective was to correlate the levels of CIC measured with two different assays in sera of patients with SLE, SSc, and RA.
Methods We used precipitation test with polyethylene glycol (PEG), molecular weight 6000 and C1q binding assay (C1q) with laser nephelometer in sera of 74 patients with SLE, 40 patients with SSc and 42 patients with RA.
Results PEG revealed abnormally high levels of CIC in 62,16% SLE sera, 45% SSc sera and 47,61% RA sera. C1q was more sensitive in SLE (78,37%) and RA (76,1%) with exactly the same result in SSc (45%).
The correlation of these two methods was weak, r – 0,089 for SLE, r – 0,22 for SSc, and r – 0,29 for RA.
Conclusion These results are not only the consequence of the imperfection of these methods but also the consequence of the enormous heterogeneity of CIC.
C1q binding assay is more sensitive, is WHO standardised, but partially gives false positive results in presence of DNA, CRP and RF, which are frequently present in sera of patients with connective tissues diseases. Besides, C1q das not detect CIC with isotypes of immunoglobulins that does not activate complement.
Precipitation test with PEG detects immunoglobulins of all isotypes, does not depend from complement and RF, and is simple and inexpensive to perform. Besides, PEG is not standardised, is less sensitive and partially gives false positive results in state of hypergamaglobulinemia, also frequently present in patients with connective tissues diseases.
This holds the allegations of WHO Working Group for the necessity of using more methods for detecting CIC from the single sample sera.