In human autoimmune diseases, low plasma levels of complement factors C3 and C4 are commonly used as a proxy for complement activation. The measurements of C3 and C4 concentrations (the result of synthesis and consumption) however, show low sensitivity in patient follow-up. We find that the estimation of the C3dg fragment released during complement activation is a better parameter for complement activation. Available techniques for measuring the activation fragment C3dg, e.g. immune-electrophoresis or involving PEG-precipitation, are time-consuming and difficult to standardize. Here we examine the specificity and use of an antibody with mono-specificity for a neoepitope at the N-terminus of C3dg, which is only exposed after cleavage of C3. We present a stable, reproducible, and easy-to-use, time-resolved immunoassay with specificity for C3dg that can be used to directly evaluate ongoing complement activation. We demonstrate that the assay can be applied to clinical

samples with a high specificity (95%) and a positive likelihood ratio of 10. It can also differentiate the complement related disease Systemic Lupus Erythematosus from controls and other immune-mediatedimmune mediated diseases like Rheumatoid Arthritis (86% specificity) and Spondyloarthritis (91% specificity). Further, we establish how the assay may also be used for experimental research in in vivo mouse models.

在人类自身免疫疾病中，低血浆水平的补体因子C3和C4通常被用作补体激活的代表。然而，对C3和C4浓度的测量（合成和消耗的结果）显示出对患者随访的敏感性较低。我们发现，补体激活过程中释放的C3dg片段的估计是补体激活的更好参数。测量激活片段C3dg的可用技术，例如免疫电泳或涉及PEG沉淀，既费时又难以标准化。在这里，我们研究了在C3dg的N端对新表位具有单特异性的抗体的特异性和用途，该抗体仅在C3裂解后才暴露。我们提供稳定，可复制且易于使用的产品，对C3dg具有特异性的时间分辨免疫测定，可用于直接评估进行中的补体激活。我们证明该测定法可用于临床

具有高特异性（95％）和阳性似然比为10的样本。它还可以将补体相关疾病系统性红斑狼疮与对照组以及其他免疫介导的免疫介导的疾病（如类风湿关节炎（86％特异性）和脊椎关节炎（91％））区分开来特异性）。此外，我们确定了该测定法还可以如何用于体内小鼠模型的实验研究。