What is the preferred specimen for most drug assays?

• A. Serum
• B. Urine
• C. Plasma
• D. Whole blood

Answer: (A)

When you want an accurate measure of how much drug is circulating in the body, a blood-derived, cell-free liquid is the most reliable matrix. Serum fits that need because it is produced when blood clots and is then separated from the clot. This yields a clean, uniform liquid with the drug concentration that reflects the circulating level, and it is a matrix that many assays (immunoassays and many chromatography methods) are validated for. It also avoids potential interference from clotting factors and anticoagulants that can affect measurements.

Urine, while excellent for detecting exposure or metabolites, varies with hydration, renal function, and time since the last dose, so it doesn’t reliably represent current circulating drug levels. Plasma can be used, but some assays are specifically validated for serum, and removing the anticoagulant-related variables helps standardize results across tests. For these reasons, serum is the preferred specimen for most drug assays.