When evaluating a potentially poisoned patient, there is no substitute for a thorough history and physical examination. Clues from the examination are generally more likely to be helpful than a “shotgun” laboratory approach that involves indiscriminate testing of blood or urine for multiple agents. When used appropriately, diagnostic tests may be of help in the management of the intoxicated patient. When a specific toxin or even class of toxins is suspected, requesting qualitative or quantitative levels may be appropriate. The purpose of this review is to examine the role of common diagnostic tests in the evaluation of the poisoned patient.