The rule in D.C. is that you need to report a work injury, or give notice, as its called, within 30 days or you can't get workers comp benefits other than medical treatment.
The D.C. Court of Appeals just ruled that the 30 days to give notice doesn't start until you are reasonably aware of the severity of the injury, its capacity to affect your ability to work and the relationship between the work injury and the accident at work. So you still may have time to report your injury if its gotten worse and you can't work because of it.
For example, a steamfitter who feels a sharp pain in his shoulder after pulling pipe but shrugs it off and goes back to work, can still get workers comp benefits if it gets worse later on and he can't work (as long as the doctor connects the shoulder problem to pulling pipe).
I wouldn't delay reporting any work injury though - its usually a red flag to insurance adjusters who will use it to deny your workers comp benefits.