There are three criteria that a veteran must meet in order to receive service-connected compensation from the Veterans Administration (VA):

1. A Current Disability. There must be a current disability, one that has been diagnosed by a medical professional;
2. An Event In Service. There must be evidence in the military service records of the occurrence or aggravation of a disease, injury, or event in service; and,
3. Medical Nexus Evidence. There must be a nexus, or link, between the current disability and the in-service disease, injury or event; i.e., the veteran must present medical evidence that the current disability and the in-service disease, injury or event are related. See Epps v. Gober, 126 F.3d 1464, 1468 (Fed. Cir. 1997); Caluza v. Brown, 7 Vet. App. 498, 506 (1995).

Medical nexus, or service connection, basically it means that the evidence establishes that a particular injury or disease resulting in disability was incurred coincident with service in the Armed Forces, or if preexisting such service, was aggravated therein. The standard of proof by which a veteran needs to show that his current medical condition is related to his military service is a medical opinion that “it is as likely as not”. This literally means the reviewing doctor need only opine that there is a 50-50 likelihood of a relationship between the veteran’s current medical condition and his or her military service. See 38 U.S.C. § 5107(b), also known as the “benefit of the doubt” rule or standard. This is a much lower standard than in the usual standard for proving disability employed in the civilian world, and one that non-VA physicians may not be aware.

Evidence relating to a veteran’s claim is subject to the “benefit of the doubt” rule, codified at 38 U.S.C. § 5107(b) as follows:

The Secretary shall consider all information and lay and medical evidence of record in a case before the Secretary with respect to benefits under laws administered by the Secretary. When there is an approximate balance of positive and negative evidence regarding any issue material to the determination of a matter, the Secretary shall give the benefit of the doubt to the claimant.

Under this provision, where the pertinent evidence is in relative equipoise, or “approximate balance,” a claimant enjoys the benefit of the doubt and his or her claim for service connection will be granted.

Medical nexus may also be shown by “continuity of symptomatology,” whereby a veteran can  satisfy the medical nexus criteria if he or she can demonstrate treatment for a particular disabling condition in the service via medical evidence, and can also show records of continuous treatment for the condition into the present. See 38 U.S.C. § 1101(3); see also 38 C.F.R. §§ 3.303(b), 3.309(a) (2008).