Indian discus thrower Neelam Jaswant Singh is the first athlete to test positive at the World Championships.
Singh tested positive in competition in Helsinki on 7 August for the prohibited stimulant pemoline.
She faces a two-year ban from competition and is provisionally suspended pending a hearing by her national federation.
Singh managed a best throw of 56.70m in Helsinki and failed to qualify for the discus final.
As of Thursday, the International Association of Athletics Federations had tested 689 samples and had no positive results.
The governing body of athletics said before the competition that it wanted to complete 850 tests from the 1,891 athletes before Sunday's closing ceremonies.