Cut-off value of C1-inhibitor function for the diagnosis of hereditary angioedema due to C1-inhibitor deficiency

Hereditary angioedema caused by C1-inhibitor (C1-INH) deficiency (HAE-C1-INH) is a rare autosomal dominant disease. Primary care physicians sometimes face difficulties in diagnosing HAE-C1-INH owing to fluctuations in C1-INH function levels influenced by blood sampling conditions. International major guidelines do not stipulate a cut-off value of C1-INH function for the diagnosis. We aimed to explore the distribution of C1-INH function levels in patients with HAE-C1-INH and elucidate the influence of blood sampling conditions using healthy volunteers' samples to confirm the cut-off value of C1-INH function. In 48 patients with HAE-C1-INH who visited the Juntendo University Hospital in Japan between 2013 and 2019, C1-INH function levels were evaluated for 160 samples during symptom-free periods and 147 samples during an acute attack. Fluctuations of C1-INH function level were also evaluated for 8 healthy volunteers, wherein the samples were divided into 3 groups according to different sampling conditions. C1- INH function levels in all patients with HAE-C1-INH were found to be < 50%. The average C1- INH function level in healthy volunteers measured soon after blood collection in an appropriate sampling condition was 77% (61-92%) with some having lower C1-INH function levels than the reference value. C1-INH function levels fluctuated unstably in inappropriate sampling conditions. In conclusion, we can confirm that a < 50% C1-INH function level can be used as the diagnostic cut-off value for HAE-C1-INH. Moreover, it is necessary to repeat measurements of C1-INH function level in appropriate blood sampling conditions to accurately diagnose HAE-C1-INH.