An evaluation of nephelometry and complement-based tests for the detection of circulating immune complexes.

Sera from 20 normal adult control subjects and 28 patients suffering from various diseases which may be associated with an immune complex disorder were investigated, using three different techniques for detection of circulating immune complexes (CICs). The sera from the patients were assigned expected positive or negative ratings by the clinicians according to clinical and laboratory criteria. This information as well as the diagnoses was withheld until the results of immune complex determinations were available. The three tests used to detect CICs were laser nephelometry (LN), 125I-C1q binding and measurement of the C3 breakdown product C3c. Serum levels of the complement components C3 and C4 were assessed on the serum specimens from the patients. Results obtained from normal control sera showed that 18 of the 20 and all 20 were negative with the C1q binding technique and LN respectively. Of 16 sera for which a positive result was expected, 5 (31,3%) and 14 (87%) were positive when examined by the C1q binding technique and LN respectively; C3c determination produced no positive results. No false-negative results were obtained with the C1q binding and C3c tests, but 2 out of 16 (13%) results obtained with the LN test were false negative. LN is a rapid, sensitive test for the detection of CICs.