In Know Who You Want To Sue, medical malpractice expert witnesses at Medical Opinions Associates write:
The decision about what medical specialist to use in a medical malpractice case is up to the client. Nevertheless, we have all too often discussed cases with individuals and attorneys when they did not know what type of medical specialist they wanted to be involved. Sometimes we get the comment that any medical expert will do. In our view, the notion that any medical expert will do is wrong-headed and possibly even self-defeating.
You need to know who you are suing. You want to have a physician expert in the same medical specialty review the records (there are exceptions, but we won’t get into them here). In this way, you ensure that the medical expert will be able to authoritatively comment on the same standards of care owed by a provider in the same specialty. It does no good to have someone of lesser expertise comment on those standards of care because the opinion will surely be impeached for lack of qualifications. That is fairly obvious. However, the same applies for someone who is overqualified because the offending provider cannot be held to higher standards of care than those owed by someone in his medical specialty. There is abundant case law that backs this assertion.