Most/many chronic conditions cannot be measured, defined, or quantified. The sufferer can give you some estimate on some scale that you describe that relates to the intensity and nature of their experiences.

I'm very glad to have seen an increasing number of health care professionals say things along the lines of "we have no idea what is causing their experience, and in fact, we think most it may be entirely psychogenic, but that doesn't change the fact that this is what they are actually experiencing, and that's as real for them as a puncture wound or an infection."