Chen finished fourth in the women’s 100m butterfly final on Sunday and was scheduled to swim in the 50 freestyle heats on Friday in Rio.
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They added: “The CSA has taken this matter seriously and demanded full cooperation from Chen in the investigation”.
Doping has cast a long shadow over swimming in the Rio Games.
“I’ve been tested four times after entering the Olympic Village”, she told Bulgarian television.
Diuretics can help to mask other stronger banned performance enhancing substances.
“If the assertion is true, the CSA will earnestly implement anti-doping regulations and safeguard legal interests according to law”, Xinhua cited an unnamed CSA official as saying in a statement. “We will cooperate with the Court of Arbitration of Sport during its investigation and will respect the final ruling by the CAS”.
The World Anti-Doping Agency (WADA) said in March that it would investigate allegations that Chinese swimming covered up positive tests ahead of Olympic trials.
Efimova was allowed to compete at the Games only after winning an appeal against a ban for a previous doping offence.
The CSA were incensed after Australian 400m Freestyle gold medalist Mack Horton called silver medalist swimmer Sun Yang a “drug cheat” in reference to his three-month ban in 2014 after testing positive for trimetazidine.
It was reported that a total of five Chinese swimmers tested positive, with two coming in October and three at the turn of the year.
“[Results] of the three of the samples were negative – it’s an incredibly big shock”.
The 19-year-old is chasing five golds in Rio, and few would bet against the 1.45m (4ft 9in) Texan adding to her tally in the apparatus finals to come. But no details have been given on what basis the swimmer could be challenging the failed test.
Analysis of a B-sample is taking place before this is confirmed by CAS.
Norandrosterone is one of the most popular steroids detected in failed tests by weightlifters.
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Olympic hosts Brazil have also been criticised by WADA after halting its drug testing programme during the build-up to Rio 2016.
