Deriving an item value from a structured file in Zabbix
If your Zabbix agent is 2.2 or later, you could use Zabbix agent item vfs.file.regexp like so:

vfs.file.regexp[/proc/meminfo,^Dirty.*([0-9]+),,,,\1]


If your Zabbix is 3.4 or later, you can use a master item to collect the data blob (perhaps with vfs.file.contents) and then extract and parse the desired values with item value preprocessing.