"I have been informed by the U.S. Anti-Doping Agency that a sample I gave at the U.S. Olympic Trials contains an elevated testosterone/epitestosterone level," Dunn said in a statement.
"While I work with USADA to resolve this matter, I am withdrawing from my relay pool position for the 2012 Olympic Games."
Dunn finished fourth in the 400 metres at the U.S. trials last month and was a member of the U.S. 4x400 metres relay pool announced this week by the U.S. Olympic Committee (USOC). She won the 2010 world indoor 400m championship.
USADA officials confirmed her test was being reviewed.
"USADA appreciates Ms. Dunn voluntarily removing herself from the Olympic team while the full facts surrounding her elevated T/E ratio and adverse carbon isotope ratio analysis (CIR) are evaluated," Travis Tygart, the anti-doping agency's chief executive, said in a statement.
"We are currently processing the B sample, and as in all cases all athletes are innocent until and unless proven otherwise through the established full, fair legal process."
USOC officials declined immediate comment.