Which specimen is most appropriate for an initial investigation when drug toxicity is suspected?

When assessing drug toxicity, an initial investigation typically aims to determine the concentration of the drug in the system at the time symptoms occur. In this context, a peak blood sample is the most appropriate choice as it reflects the maximum concentration of the drug in the bloodstream shortly after administration, providing critical information about drug levels when toxicity is suspected.

Obtaining a peak sample can help discern if the drug level is within therapeutic ranges or significantly elevated, correlating with the patient's clinical presentation. Timing is essential in this scenario, as substances often exhibit peak levels shortly after administration, which can be crucial for accurate diagnosis and management.

The other specimen types, such as trough samples, urine, or gastric fluid, may not provide the most timely or relevant information about the immediate toxicological state. A trough sample measures levels just before the next dose is due and may not reflect an active state of toxicity. Urine collected at the time of symptoms might not reflect current blood concentrations and could yield delayed results. Gastric fluid could be indicative of recent ingestion, but it may not accurately represent the overall systemic exposure to the drug. Therefore, a peak blood sample stands out as the most informative for initial investigations in suspected drug toxicity cases.