The 35-year-old former Portugal international failed the test after Fluminense's 2-0 victory over Boavista in Brazil's Campeonato Carioca March 30.
In a brief statement Wednesday, he blamed contaminated vitamins for the positive sample and said he would await results of a secondary test Friday before commenting further.
"I have spoken to my lawyer and I will wait for the results of the B sample," Deco was quoted as saying by Xinhua. "I have always taken vitamins and have never used anything illicit.
"I've been playing professional football for 18 years, including 15 in Europe, without any problems. I will make a decision only after seeing the B result and I will stop at nothing to clarify the facts."
Deco has been left out of Fluminense's squad for Thursday's Copa Libertadores round of 16 first leg against Emelec in the Ecuadorian city of Guayaquil.
The star has helped Fluminense to two Brazilian Series A titles since joining the club in 2010.
He is one of few players to have won the Champions League with two different clubs, having claimed the winners' medal for Porto in 2004 and Barcelona in 2006.