detect which members read a certain topic?

[kh99]

There is a table called threadread:

	userid int(10) unsigned NOT NULL default '0',
	threadid int(10) unsigned NOT NULL default '0',
	readtime int(10) unsigned NOT NULL default '0',
	PRIMARY KEY  (userid, threadid),
	KEY readtime (readtime)

but I don't know for sure if it stores every thread read by any user ever.