How often do you rely on using the C1 inhibitor functional assay versus the quantitative level alone to diagnose HAE?

We have seen a significant increase in lab costs billed to well-insured patients for some of the more "esoteric", detailed testing associated with immune/inflammatory disorders such as HAE. The cheapest screen is a C4 level, which, if normal during an active angioedema episode, makes C1 inhibitor deficiency highly unlikely. However, it is quite common that the patient presents with days or even weeks since the last episode of swelling. I use the functional assay because it will be abnormal in bo...

Should always use both since the functional assay distinguishes between Type 1 and Type 2 HAE.

An interesting question. I have only ever ordered both C1 Inh level and function, along with C4, and not just the level. The majority of HAE patients will have type I HAE, with low quantitative levels, while a minority (about 15%) will have normal levels but dysfunctional protein, or type II. Thus, statistically, measuring C1 Inh level would be more likely to pick up HAE than function.

Until reviewing UpToDate, I was not aware that some centers start with C4 and C1 Inh levels first. The reasonin...