MLB.com's William Ladson reports that the deal is a one-for-one trade, however, reports from MLB Daily Dish's Chris Cotillo and the Washington Post's Adam Kilgore have the Rays trading three players to Washington for Karns.
Lobaton, 29, has spent parts of the last three seasons with the Rays, posting a .230/.315/.349 batting line in 547 plate appearances. He was originally signed as an amateur free agent out of Venezuela by the San Diego Padres in 2002.
He split the team's catching duties with Jose Molina last year. He's been an overall positive defensively, but he's thrown out just 22 of 136 career base stealers, and his prowess in the batter's box isn't likely to land him a full-time role in Washington. Wilson Ramos projects as the Nats' regular backstop.
Karns, a 26-year-old right hander, reached the majors for the first time last season after putting together a solid minor league campaign from 2011 to 2013. In 304 innings across five levels of the Nats' system, Karns managed a 2.66 ERA with 362 strikeouts and 128 walks.
He's somewhat old for a prospect, but his 98 MPH fastball and strong strikeout rate could be enough to make him an interesting addition to the Rays organization. His development was hindered by a torn labrum in 2010.