It has only been in recent history that a national criminal background check can be performed instantly through a site like CriminalBackgroundRecords.com.  Although, as the article explains, the current national background check being offered is not in fact a true 50 state national background check.

Currently there is not a full 50 state criminal background check available in the USA.  However, the press release does outline the history of the gradual integration of new states into the national background check; which has caused the national search to become closer and closer to a true 50 state criminal background check.

The current instant national background check available today does include a true 50 state sex offender registry search for free in addition to the standard 46 state criminal conviction search included with a national background check ordered online today at CriminalBackgroundRecords.com  (including felonies and some misdemeanors).

Full article:  National Background Check