C1 inhibitor function using contact‐phase proteases as target: evaluation of an innovative assay

Background Controlling prekallikrein activation by C 1 inhibitor ( C 1 I nh) represents the most essential mechanism for angioedema patient protection. C1 I nh function in the plasma is usually measured based on the residual activity of the C 1s protease not involved in the pathological process. We have hereby proposed an alternative enzymatic measurement of C 1 I nh function based on contact‐phase activation and correlation with angioedema diagnostic requirements. Methods The contact phase was reconstituted using the purified components, with C 1 I nh standard or plasma sample. The kinetics of the amidase activity were monitored using P ro‐ P he‐ A rg‐ p NA , independently of alpha2‐macroglobulin. We prevented any interference from a possible high plasma kininogenase activity by preincubating the samples with protease inhibitor. Receiver operating characteristics ( ROC ) were used to calculate the assay's diagnostic performance. Results The calibration curve was built using C 1 I nh standard (threshold limit 0.10 × 10 −3 U, i.e., 0.2 pmol), and C 1 I nh function was quantified in the sample, with a reference interval established based on healthy individuals ( n  = 281; men: 0.61–1.10 U/ml, median: 0.85 U/ml; women: 0.42–1.08 U/ml, median: 0.74 U/ml). The median values of female donors were lower than those of the others due to estrogen, yet C 1Inh function remained within the reference interval. The ROC curve calculation provided the following optimum diagnostic cutoff values: women 0.36 U/ml (area under curve [ AUC ]: 0.99; sensitivity: 93.48%; specificity: 99.37%); and men 0.61 U/ml ( AUC : 1; sensitivity: 100.0%; specificity: 100.0%). Conclusion The performance outcome provided features suitable for angioedema diagnostic or follow‐up. Established by means of the kinin formation process, this assay should be preferred over the method based on a C 1s protease target.