indexterm element not supporting keyref?

[#1]

I was wondering if there is a way to include keyref element(s) inside an indexterm element, e.g. 

<indexterm>Regsiter <keyword keyref="sample_lower"></keyword> data </indexterm>

where the keyword "sample_lower" has been defined in a variable file. It is to note that the same keyref works perfectly outside an indexterm element.