The determination of brain death is based on a comprehensive clinical assessment. A confirmatory test-at least, in adult patients in the United States-is not mandatory, but it typically is used as a safeguard or added when findings on clinical examination are unwontedly incomplete. In other countries, confirmatory tests are mandatory; in many, they are optional. These tests can be divided into those that test the brain's electrical function and those that test cerebral blood flow. A false-positive result (i.e., the test result suggests brain death, but clinically the patient does not meet the criteria) is not common but has been described for tests frequently used to determine brain death. A false-negative result (i.e., the test result suggests intact brain function, but clinically the patient meets the criteria) in one test may result in more confirmatory tests and no resolution when the test results diverge. Also, pathologic studies have shown that considerable areas of viable brain tissue may remain in patients who meet the clinical criteria of brain death, a fact that makes these tests less diagnostic. Confirmatory tests are residua from earlier days of refining comatose states. A comprehensive clinical examination, when performed by skilled examiners, should have perfect diagnostic accuracy.