38 CFR §3.326

1. (a)Where there is a claim for disability compensation or pension but medical evidence accompanying the claim is not adequate for rating purposes, a Department of Veterans Affairs examination will be authorized. This paragraph applies to original and supplemental claims as well as claims for increase submitted by a veteran, surviving spouse, parent, or child. Individuals for whom an examination has been scheduled are required to report for the examination.
2. (b)Provided that it is otherwise adequate for rating purposes, any hospital report, or any examination report, from any government or private institution may be accepted for rating a claim without further examination. However, monetary benefits to a former prisoner of war will not be denied unless the claimant has been offered a complete physical examination conducted at a Department of Veterans Affairs hospital or outpatient clinic.
3. (c)Provided that it is otherwise adequate for rating purposes, a statement from a private physician may be accepted for rating a claim without further examination.