Quantification of human C1 esterase inhibitor protein using an automated turbidimetric immunoassay

BACKGROUND: Impaired levels or function of C1 inhibitor (C1‐INH) results in angioedema due to increased bradykinin. It is important to distinguish between angioedema related to C1‐INH deficiency and that caused by other mechanisms, as treatment options are different. In hereditary (HAE) and acquired (AAE) angioedema, C1‐INH concentration is measured to aid patient diagnosis. Here, we describe an automated turbidimetric assay to measure C1‐INH concentration on the Optilite(®) analyzer. METHODS: Linearity, precision, and interference were established over a range of C1‐INH concentrations. The 95th percentile reference interval was generated from 120 healthy adult donors. To compare the Optilite C1‐INH assay with a predicate assay used in a clinical laboratory, samples sent for C1‐INH investigation were used. The predicate results were provided to allow comparison. RESULTS: The Optilite C1‐INH assay was linear across the measuring range at the standard sample dilution. Intra and interassay variability was <6%. The 95th percentile adult reference interval for the assay was 0.21‐0.38 g/L. There was a strong correlation between the Optilite concentrations and those generated with the predicate assay (R (2 )= 0.94, P < 0.0001, slope y = 0.83x). All patients with Type I HAE (n = 24) and AAE (n = 3) tested had concentrations below the measuring range in both assays, while all patients with unspecified angioedema (UAE), not diagnosed with HAE or AAE had values within the reference range. CONCLUSION: The Optilite assay allows the automated and precise quantification of C1‐INH concentrations in patient samples. It could therefore be used as a tool to aid the investigation of patients with angioedema.