A determination as to disability under either title II or title XVI of the Social Security Act requires objective medical evidence from an acceptable medical source to establish the medically determinable impairment. The evidence must also show the nature and extent of the impairment(s) during the time disability is alleged and describe the individual’s ability to function despite the impairment(s).

It ordinarily must first be determined whether the claimant has an impairment which meets the specific requirements of any condition described in the Listing of Impairments. The time-tested source of reference for medical evidence is the Listing of Impairments. However, a determination of “equals” necessarily requires consideration of medical findings beyond those reflected in any specific section of the Listings.

Even more definitive and detailed medical documentation is frequently required to make a judgment about the claimant's residual functional capacity, when vocational factors must be considered to evaluate his or her disability.

Medical reports containing descriptions of examinations or treatment of the individual are basic to the determination of the onset of disability. The medical evidence serves as the primary element in the onset determination. Reports from all medical sources and entities that maintain medical sources’ evidence (e.g., physicians, hospitals, and government agencies) which bear upon the onset date should be obtained to assist in determining when the impairment(s) became disabling.

With slowly progressive impairments, it is sometimes impossible to obtain medical evidence establishing the precise date an impairment became disabling. Determining the proper onset date is particularly difficult, when, for example, the alleged onset and the date last worked are far in the past and adequate medical records are not available. In such cases, it will be necessary to infer the onset date from the medical and other evidence that describe the history and symptomatology of the disease process.

Particularly in the case of slowly progressive impairments, it is not necessary for an impairment to have reached listing severity (i.e., be decided on medical grounds alone) before onset can be established. In such cases, consideration of vocational factors can contribute to the determination of when the disability began. (See DI 25501.200 - DI 25501.470 for information about determining the established onset date.)