Routine urine toxicologic immunoassays have limited sensitivity, even for common drugs, and a "negative" urine drug screen should not be used to exclude drug intoxication, and a "positive" urine drug screen cannot be used to assess the extent or degree of intoxication.

For example, a typical opiate screen does not reliably identify oxycodone and hydrocodone and does not identify synthetic opioids such as fentanyl or buprenorphine, and a typical benzodiazepine screen does not reliably identify clonazepam.

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Delays in gastric emptying and gut hypomotility may result from fasting status, from overdose itself, or from coingestion of opioids or anticholinergic drugs, and controlled release drugs have a prolonged absorption phase [15, 16].

Hypoperfusion of the gastrointestinal tract, secondary to hypotension and/or splanchnic vasoconstriction, can slow absorption [17]. Enterohepatic recirculation may play a role in elimination of certain drugs.

If a patient was exposed to an exceedingly large quantity of drug or toxin, 3% of the original dose could potentially still have clinical effects.

In addition, the pharmacokinetics of many drugs will be altered in patients with organ failure [12, 13].

Mechanisms of metabolism may be saturated in overdose [14].

As an example of prolonged half-life in overdose, many references indicate the half-life of baclofen is approximately 2–4 h, but in overdose, the duration of effect far exceeds the recommended five half-life calculation [3].

In contrast, a "positive" urine drug screen by itself is not confirmatory, but in the setting of an appropriate history, clinical presentation and physical examination can support intoxication.

Although most hospital laboratories can readily measure serum concentrations of some common drugs in overdose, including lithium, digoxin, phenobarbital, phenytoin, and valproic acid, there are many drugs that cannot be measured in a clinically relevant time frame.

As an alternative, the SDS should include a referral mechanism through which advice on treatment is available from a physician board-certified in medical toxicology.