Associating Regular Expression Extractor reference name with thread number

I've a problem I can't find an answer to; I'm hoping someone more knowledgeable than I can give me a pointer or two. Here is the background:

I'm running a script in jmeter 2.9 that currently has one "user" (i.e., thread). The script makes an API call that returns a numeric value that is stored in a (reference name) identity by the Regular Expression Extractor; currently, the name is hardcoded to "CAT1ID". The value of this identity (i.e., ${CAT1ID}) is used in a later portion of the script. For a single modeled user, the script works exactly as intended. I wish to increase the number of users (i.e., threads) and have unique identities associated with each user. For example, if I have three threads, I'd like to have CAT1ID, CAT2ID, and CAT3ID and I wish to later pass the values of these identities to other portions of the script. I'm requesting help with the following:

1). How should the reference name identifier be written? I tried CAT${__threadNum()}ID but I'm not certain this is working correctly.
2). How should I reference this identifier in subsequent calls?

Thank you very much,

Bohdan Bodnar