Complement diagnostics: concepts, indications, and practical guidelines

Abstract

Aberrations in the complement system have been shown to be direct or indirect pathophysiological mechanisms in a number of diseases and pathological conditions such as autoimmune disease, infections, cancer, allogeneic and xenogeneic transplantation, and inflammation. Complement analyses have been performed on these conditions in both prospective and retrospective studies and significant differences have been found between groups of patients, but in many diseases, it has not been possible to make predictions for individual patients because of the lack of sensitivity and specificity of many of the assays used. The basic indications for serological diagnostic complement analysis today may be divided into three major categories: (a) acquired and inherited complement deficiencies; (b) disorders with complement activation; (c) inherited and acquired C1INH deficiencies. Here, we summarize indications, techniques, and interpretations for basic complement analyses and present an algorithm, which we follow in our routine laboratory.

Figure 1

Overview of the complement system. Recognition by the lectin and classical pathways leads to the assembly of the C4b2a convertase, which cleaves C3. This reaction is greatly amplified by the alternative pathway, generating more C3b, and ultimately initiating the terminal sequence. The fluid phase anaphylatoxins, C3a and C5a, together with the cell-bound opsonins iC3b and C3d,g, facilitate phagocytosis. The main inhibitor of each step in the cascade is indicated in boxes: C1INH for initiation, C4BP for the classical pathway, factor H for the alternative pathway, and CD59 for the terminal sequence.

Figure 2

Activation and consumption of complement in vivo and in vitro. In vivo, complement is activated, and C3 gives rise to the activation products C3a, C3b/iC3b, C3d,g, and C3c (indicated by arrows). In vivo, a fraction of these complement products are bound to and eliminated by different complement receptor-bearing cells in contact with plasma (a). When blood is drawn in the presence of EDTA, all further complement activation is inhibited (b). The complement system is active in serum and may be activated to a substantial degree in vitro in maltreated samples (c), but it can be kept essentially intact in properly handled samples (d). The thickness of the arrows in each panel indicates the degree of C3 cleavage.

Figure 3

Algorithm for complement analyses. The aims are to diagnose complement deficiency in patients with recurrent bacterial infections (a), diagnose the cause of their persistent complement activation (b), and to dissect the cause of C1INH deficiency (c). See text (Section 5.1) for details.

Figure 4

Complement activation and hemolytic function during an SLE exacerbation. C3, C4, factor B, and other components are consumed, leading to a depression in hemolytic function (red line). The resulting activation products, C3d,g, C3a, and sC5b-9 (green line), peak concomitantly with the SLE disease index (SLEDAI).