20 C.F.R. § 220.176
There is a statutory requirement that, if an annuitant is entitled to a disability annuity, the annuitant's continued entitlement to such an annuity must be reviewed periodically until the employee or child annuitant reaches full retirement age and the widow(er) annuitant reaches age 60. When the annuitant is entitled to a disability annuity as a disabled employee, disabled widow(er) or as a person disabled since childhood, there are a number of factors to be considered in deciding whether his or her disability continues. The Board must first consider whether the annuitant has worked and, by doing so, demonstrated the ability to engage in substantial gainful activity. If so, the disability will end. If the annuitant has not demonstrated the ability to engage in substantial gainful activity, then the Board must determine if there has been any medical improvement in the annuitant's impairment(s) and, if so, whether this medical improvement is related to the annuitant's ability to work. If an impairment(s) has not medically improved, the Board must consider whether one or more of the exceptions to medical improvement applies. If medical improvement related to ability to work has not occurred and no exception applies, the disability will continue. Even the medical improvement related to ability to work has occurred or an exception applies (see § 220.179 for exceptions), in most cases the Board must also show that the annuitant is currently able to engage in substantial gainful activity before it can find that the annuitant is no longer disabled.
20 C.F.R. §220.176