Radial Immunodiffusion Method for Evaluation of C1-Esterase Inhibitor Function

Rationale: To examine the frequency of abnormal C1-esterase inhibitor (C1-inh) function in samples with low C1-inh level during a 2 year period at a single Canadian centre that utilizes radial immunodiffusion (RID) for C1-inh function, as opposed to the chromogenic assay. 

Methods: Retrospective analysis of all C1-inh function samples submitted to a tertiary lab and analyzed via RID and turbidimetry methods.  

Results: 1438 samples were assessed for C1-inh function from July 2013 - June 2015. 558 were assessed for C1-inh function and levels.  Of these, 545 (97.7%) had normal C1-inh function and level and 1 (0.18%) had abnormal C1-inh function and a low level. Nine (1.6%) had normal C1-inh function and a low level. Of these, 3 had levels <50% of the lower limit of normal, consistent with possible hereditary angioedema (HAE) type II. 

Conclusions: C1-inh function should be abnormal when C1-inh level is less than 50% of the lower limit of normal. Three samples (75%) with C1-inh level less than 50% of normal had normal C1-inh function, indicating the RID technique may result in falsely normal C1-inh function. In some labs, C1-inh function is the initial screening test for Type I HAE hence, both type 1 HAE and Type 2 HAE may be missed using RID alone. These samples may need to be reassessed by another method such as the chromogenic assay to rule out both HAE type I and II.