Dinamo Zagreb have confirmed Arijan Ademi failed a doping test but expressed their surprise at the results.
Dinamo Zagreb have confirmed that midfielder Arijan Ademi tested positive in a routine doping exam following the Champions League victory over Arsenal.
The Croatian champions have stated that UEFA have opened a procedure in relation to the 24-year-old's test, though they have refused to comment on the duration of the investigation.
Dinamo did, however, express their surprise at the positive result given Ademi and the club's clean record with testing in recent years.
"We are aware of the results of the doping control test after the match of the Champions League first round against Arsenal and that our player, Arijan Ademi, was positive, and that UEFA opened proceedings against him," read an official statement released by the club.
"According to UEFA rules, Dinamo cannot comment on the case until the B sample is given.
"For years, our players have been regularly subjected to doping controls and nothing like this has ever happened, so we have to say that we are surprised by the result.
"Arijan Ademi was tested six times last year and each time the result was negative. Players have also been tested several times this season by UEFA and the Croatian Anti-Doping Agency and everyone was always negative."
Croatia international Ademi has played 13 times in all competitions for Dinamo this season, including the full 90 minutes of the shock 2-1 win over Arsenal last month.
UEFA announced earlier this month that it was launching "the strongest anti-doping programme ever seen in European football" with a greater degree of co-ordinated testing in the Champions League in particular.