Right now, it seems that "passionate love" is measured in a discrete scale based on answers to a questionnaire. The "Passionate Love Scale" (PLS) is mentioned in this blog post and was introduced by this article in 1986.

In my other reply to my original comment I showed a study that finds that "high levels of passionate love of individuals in the early stage of a romantic relationship are associated with reduced cognitive control", in which they use the PLS.