A biological compound or attribute that provides evidence of the presence of, or vulnerability to, a specific disorder. In general, two types of marker are distinguished. A state marker identifies a current abnormality that most typically reflects a transient or reactive condition of the subject, such as the degree of activity of an underlying disorder or the recent use of a drug. A trait marker identifies a relatively stable and enduring attribute that reflects a continuing condition or, particularly in the case of a genetic marker, a predisposition to a specific disorder.

Most biological markers for alcohol and other drugs are state markers, and many simply reflect the recent history of consumption. A high blood alcohol level, for example, may identify a state of alcoholic intoxication, but it does not confirm alcohol dependence. Many, but not all, state markers for alcohol are in fact tests of hepatic damage (such as elevated plasma ?- glutamylfransferase). They are diagnostic tests of alterations in liver status secondary to chronic drinking, and not valid indicators of alcohol dependence. Other biological state markers for heavy alcohol consumption include de- sialotransferrin and acetaldehyde-protein adducts or antibodies to them.