The workers comp insurance shouldn't deny your claim just because you have a pre-existing condition, but many times they do.
In D.C. an underlying condition that was made symptomatic or aggravated by an injury at work is covered by workers compensation. Many workers have had prior on the job injuries or conditions, like a bad back, that have flared up from time to time.
Some people have medical conditions that may make an injury more severe than if it had happened to someone else, like a condition that causes brittle bones. A fracture caused by a fall at work may be more severe and require additional treatment, surgery and recovery time than it would have otherwise.
If you have a pre-existing condition or medical history and you are hurt at work, don't give the insurance company a recorded statement. They will use any mention of your condition to deny or delay your workers comp benefits - medical treatment and weekly income - even if there is no medical basis to do so.