Total Haemolytic Complement Activity at Diagnosis as an Indicator of the Baseline Activity of Antineutrophil Cytoplasmic Antibody-associated Vasculitis

OBJECTIVE: The total haemolytic complement activity (CH50) assay evaluates the functioning of the complement system Accumulating evidence indicates that the activation of the complement system plays a critical role in the pathogenesis of antineutrophil cytoplasmic antibody-associated vasculitis (AAV) Therefore, this study aimed to investigate whether CH50 levels at diagnosis could reflect the baseline activity of AAV. METHODS: This retrospective study included 101 immunosuppressive drug-naïve patients with AAV At diagnosis, all patients underwent clinical assessments for disease activity, including measurement of the Birmingham Vasculitis Activity Score (BVAS) and Five Factor Score (FFS), and laboratory evaluations, such as tests for CH50, C3, and C4 levels The association between CH50 levels and disease activity was determined. RESULTS: The median BVAS and FFS at diagnosis were 120 and 10, respectively, whereas the median CH50 level was 604 U/mL There was a negative correlation between the CH50 level and BVAS (r=−0241; p=0015) A CH50 cut-off value of 621 U/mL was used to classify the patients into two groups: patients with CH50 levels <621 U/mL (low-CH50 group) and those with CH50 levels ≥621 U/mL (high-CH50 group) The low-CH50 group had a higher proportion of patients with high disease activity, based on the BVAS, than the high-CH50 group (525% vs 238%, p=0004) Additionally, the low-CH50 group exhibited a lower relapse-free survival rate than the high-CH50 group; however, this difference was not statistically significant (p=0082). CONCLUSION: Low CH50 levels at diagnosis may reflect high baseline activity of AAV.