Laboratory testing for C1 inhibitor deficiency: a comparison of two approaches to C1 inhibitor function

C1 inhibitor deficiency may be hereditary or acquired. It is characterized by absent or poorly functioning C1 inhibitor. The disorder is rare, with prevalence estimated at 1/50,000. The very low incidence of the condition makes the sensitivity and specificity of assays used particularly important. Two different methods are commercially available to measure C1 inhibitor function. There are few data comparing these assays.

Two assays of C1 inhibitor function (C1 inhibitor-C1s complex formation or inhibition of C1 esterase cleavage of artificial substrate [colorimetric]) were compared in 71 patients (28 hereditary angioedema, 2 acquired angioedema and 41 controls).

Qualitatively, the two assays showed good correspondence (92%). Six of 71 results were discordant. Correlation in quantitative terms was moderate (R = 0.81).

Both assays show high sensitivity for hereditary/acquired angioedema. The colorimetric assay is more prone to false-positive results. However, clinical interpretation is not adversely affected.