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A tox screen is a set of standard blood tests for the most common poisons, pharmaceuticals and illegal recreational drugs. When a patient shows symptoms that indicate poisoning or overdose, a tox screen is usually the first diagnostic test ordered by the physician. However, it does have some weaknesses:

- It takes time to process the results, although the test is designed to be completed as quickly as possible.

- It does not test for common toxins that take time to test for.

- It favours environmental toxins and drugs that are likely self-administered at the expense of poisons that are usually deliberately given to a patient or those the patient may accidentally be exposed to.

As such, even a clear tox screen is not a definitive indication that the patient is not suffering from some form of poisoning.

Drug test at Wikipedia