XML_CONTENT_BASED_ROUTING will parse incoming requests and extract up to 3 elements based on the querystrings you provide. The advantage is that it handles the XML parsing, querying, and extraction for you.
It does not run against the response. You can still achieve your goal using string matching against the response data instead of XML element extraction. Since the response is your server's output, it's probably pretty predictable, making string matching more reliable than it might be on incoming requests from random clients (e.g., you don't need to worry if it's going to be instead of ).
You'll need to collect the response (some example code in HTML Comment Scrubber or HTTP Payload Collection) and then look for your string in the response data:
when HTTP_RESPONSE_DATA {
if {[HTTP::payload] contains {5}} {
implement monitor code
}
}