I did the self test in the book "Highly Sensitive Person" Elaine N. Aron.
23 is the total score in that test. I associated with 20 of them.
I am an Empath too, or at least identify with that definition.
A lot of the HSP characteristics I think are quite similar to the reactions that empaths would have. For example a reaction to over-stimulation. Sometimes it's difficult to separate and understand the two. But are these actually separate terms describing separate things? For the empath, at least in the definition and understanding of the term, there is not an attempt to classify sensitivity in terms of biology or biological differences. But a sense that empaths feel their emotions and others more keenly. But could one not be the effect of the other, inter-related. (Sorry thinking aloud here). I suppose not in all people or cases. People seem to sometimes only identify with one of these.
The person mainly behind the HSP movement, Elaine N. Aron makes a point, though, of making a clear distinction between the terms and usages of 'HSP' and 'Empath'. She's very clear that these are separate, different things, Mainly because, I think, she wants HSP to be accepted as a medical, (accepted) definition. She explains that in HSP 'high sensitivity' is due to physiological differences in the wiring of the nervous system, and how this can become over-loaded in the HSP as a result of stimulation, mainly through the senses.
However, I think if you have the two then it's difficult to differentiate the two- because there can be an overlap in behaviours / effects. I suppose if there is a difference you could think about looking to the cause, is it 'biological' as in 'HSP' as she is saying, as in say biological over-load - or due to the effects of empathy.
To be honest I'm still confused and not sure if the definitions will change over time, or be better understood and incorporated in common usage. My GP looked at me blank faced when I mentioned HSP. I get the sense that these phrases can be thought of as over used and a bit of psycho-babble, which maybe deflects from some of the usefulness of the information they contain. Sorry this post is not that well put across.