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Appropriate use of critical values improves patient outcome by ensuring that physicians are promptly notified of immediate lifethreatening conditions. Conversely, overuse squanders resources and actually may impair patient outcome. A generic critical values list derived from interlaboratory surveys is an excellent starting point, but every laboratory must customize its list to meet the needs of the organization that it serves. Category-specific and once-per-period critical values can limit superfluous reporting, but they make the critical values list more complicated. Strict semantic interpretation of critical limits is appropriate. The best ways to report critical values are by telephone and by alphanumeric pager. When required, repeat analysis should precede critical value reporting. Laboratories should avoid reporting invalid results (due to poor specimen integrity, for example) as critical values. An institutional committee initially should approve, and periodically should review and revise, the critical values policy.