IMMUNE COMPLEXES

CLINICAL UTILITY:

Circulating immune complexes (CIC) occur in patients’ sera in a wide variety of disease states and seem to play a pathogenetic role in many conditions. Assaying the serum level of the complexes often aids in establishing a diagnosis, determining the prognosis, or in monitoring therapy. In many cases, serial determinations of the serum level of CIC are helpful. When the test for immune complexes is ordered, our lab performs two C1q-binding tests on each patient sample: a fluid-phase and a solid-phase assay. Both tests detect circulating immune complexes which bind C1q; however, discrepancies between the two assays are common.

The solid phase assay is an enzyme linked immunoassay which is more sensitive to complexes containing IgG. Results from this test correlate well with disease activity in SLE.

The fluid phase test, a radioimmunoassay, detects complexes which bind C1q and are precipitable with polyethylene glycol. In contrast to the solid phase assay, it will detect immune complexes containing either IgG or IgM antibodies. This test may be helpful in evaluating patients with rheumatoid arthritis, Sjogren’s syndrome, and some forms of vasculitis, as well as patients with acute leukemia. Results of the assays are listed separately as “fluid phase C1q-binding”, in units of “percent binding”, and “C1q solid-phase”, in “micrograms aggregated human gamma-globulin-equivalents per milliliter (mcg AHG-eq/ml).”

METHOD DESCRIPTION:

C1q-binding (fluid phase): Radioimmunoassay (125I-labeled C1q binds to immune complexes present.)
C1q-binding (solid phase): Enzyme-linked-immunosorbent assay (EIA).

REFERENCE RANGE:

Fluid phase 0-9% binding
Solid phase <9 mcg AHG-eq/ml.

SPECIMEN REQUIREMENTS:

1.0 ml serum. Send on dry ice.