You only need to obtain an ABN if the service being provided is a service that is customarily covered by the Medicare plan and you are having the client pay for this service because for whatever reason it is likely to be denied.
Example ( I know this is unlikely but for example purposed only) ….client requests a flu imms in January but had already received this service in Novemeber, then the client would have to self pay the imms cost as Medicare will only cover one vaccine per season. In this case, the client would need to sign the ABN.
If a client with Medicare B requests a Tdap and chooses to self pay, since this is not covered by Medicare Part B, then no ABN is needed.