This figure is likely derived from the mathematical observation that 0.55 (50% elimination, five times) equals 0.03125, suggesting that only 3% of a drug remains following five half-lives.However, this approach may not be appropriate in every case.
Care coronary dating guide hemodynamic interpretation unit
We recommend identification of drugs or toxins by careful history and targeted testing.
An observation period of longer than five half-lives is appropriate when there is a possibility of an extremely large drug overdose, delayed drug absorption, delayed elimination, or interaction with another agent.
To determine the extent to which inaccurate brain death determination by clinical testing may occur in this setting, we conducted a review of the literature in MEDLINE and SCOPUS using the search terms "brain death mimic" and "brain death drug overdose" for the dates January 1, 1960 to June 10, 2015.
A total of 1394 titles were reviewed for relevance to the topic, and only ten case reports of brain death mimicry were found (three baclofen [3, 4], two snake bites [5, 6], and one each of valproic acid , amitriptyline , mixed diazepam ethylene glycol , bupropion , and phorate , an organic phosphorous compound)."Evidenced-Based Guideline Update: Determining Brain Death in Adults" suggests that the clinician should exclude the presence of a central nervous system (CNS)-depressant drug effect by "history, drug screen, and calculation of clearance using five times the drug’s half-life."  However, there may be limitations to this approach.
Even when the specific drugs are quantitatively identified, the use of kinetic data to determine clinical effects is limited because drugs often have prolonged half-lives in overdose.
For certain drugs and toxins, the duration of effect may extend beyond their detected presence in the vascular space.
Furthermore, the pharmacokinetics of absorption and elimination of a drug in large dose may be different than published pharmacokinetic data suggest, which are typically obtained following therapeutic dosing, generally in healthy subjects without co-exposures .
The reasons for prolonged half-lives in overdose are numerous.
Brain death is diagnosed clinically when an irreversible and proximate cause of brain injury is identified and no brain function is present upon clinical assessment [1, 2].