Nike Oregon Project athlete and 2013 World Championship 1,500-meter silver medalist Matthew Centrowitz denied any allegations of a positive test for performance enhancing drugs on Twitter, Monday afternoon.

Earlier in the day, a fan questioned Centrowitz on a rumor of a positive test. 

@xCattanach dude, are you honestly that gullible? What a joke. You obviously don't know me

— Matthew Centrowitz (@MattCentrowitz) March 3, 2014

After Centrowitz's initial comment, Flotrack contacted various sources regarding the claim. The only public discussion on the matter can be found on the LetsRun.com message boards. 

Five hours Centrowitz later added. 

Happy to say that I’m back running again after missing the indoor season due to illness..... (more)

— Matthew Centrowitz (@MattCentrowitz) March 3, 2014

...I would like to categorically dispel any ugly rumors as to why I missed a few races. I will be racing in the outdoor season as normal.

— Matthew Centrowitz (@MattCentrowitz) March 3, 2014

On Feb. 3, Flotrack reported Centrowitz would miss the 2014 indoor season due to illness. According to coach Alberto Salazar, Centrowitz contracted a "bad viral infection that needed him to take medication."