Blood samples from 15 patients with systemic lupus erythematosus (SLE) and 15 healthy blood donors were allowed to coagulate for one hour at room temperature, followed by one hour at 4 or 37 degrees C. The complement activity of the serum samples was assessed by three different functional assays. Serum samples from patients with SLE obtained by coagulation at 37 degrees C had a lower complement activity than serum samples from blood coagulated at 4 degrees C when the capacity of the serum samples to solubilise precipitable immune complexes and to support the attachment of complement factors to solid phase immune complexes was determined. Haemolytic complement activity was not affected by the coagulation temperature. The content of C1q binding immune complexes in paired serum samples obtained after coagulation at 4 and 37 degrees C was similar and the size distribution of the immune complexes, determined by high performance gel permeation chromatography, was also similar. This study shows that the results of functional complement assays, applied to serum samples from patients with SLE cannot be compared unless the conditions for blood coagulation and serum handling are defined and are the same. The data also indicate that assays measuring complement mediated solubilisation of immune complexes and the fixation of complement factors to solid phase immune complexes are more sensitive indicators of complement activity than the haemolytic assay.
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