The AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Fourth Edition, notes that pain is a subjective perception, and usually no exact relationships exist among the degree of pain, extent of pathologic change, and extent of impairment. Chapters 3, 14, and 15 regarding the musculoskeletal system, mental and behavioral disorders, and pain, respectively, may be relevant in evaluating an individual's pain following musculoskeletal injury. The eight diagnostic characteristics of chronic pain are duration, dramatization, diagnostic dilemma, drugs, dependence, depression, disuse, and dysfunction; if four of the eight are present, a presumptive diagnosis is established. The fourth edition of AMA Guides distinguishes chronic pain syndrome and psychogenic pain (the former is not considered a mental disorder), but diagnosis of pain should consider psychological factors that are judged to play a significant role in the onset, severity, exacerbation, or maintenance of the pain. Thus, two individuals may have identical musculoskeletal histories, physical exams, and pain out of proportion to their injuries, but when one of the patients has psychological factors that are known and understood, the evaluating physician may diagnose a pain disorder associated with psychological factors. Because formally associating psychological factors and diagnosis usually requires legal discovery, physicians may keep the psychological factors confidential, diagnose chronic pain syndrome, and evaluate using Chapter 15 of the fourth edition of the AMA Guides.