Urine is the most commonly collected specimen in suspected poisoning cases because it often contains higher concentrations of toxins and their metabolites for a longer duration compared to blood. Urine collection is non-invasive, easily obtainable, and reflects exposure over time, making it valuable for both acute and delayed poison detection. Blood levels may decline rapidly due to distribution and metabolism, whereas urine provides a wider window for identifying many poisons. This helps guide diagnosis and management effectively in clinical toxicology.

Reference: Goldfrank's Toxicologic Emergencies, Goldfrank LR, 12th Edition.