Which analyte is commonly preserved by chilling the sample immediately after collection?

• A. Ammonia
• B. Renin
• C. ACTH
• D. ABG

Answer: (A)

Ammonia is highly labile in blood samples because cells continue producing it after collection and, in some conditions, it can diffuse or be converted, leading to artifactual changes. Chilling the specimen immediately on ice slows down these in vitro processes and stabilizes the true in vivo concentration, so the measured ammonia reflects what was present in the patient rather than what happened in the sample after draw.

The other options don’t have this same immediate-on-ice preservation rule. Arterial blood gas samples are sensitive to cooling because gas solubility and pH can be altered, so they require prompt handling rather than chilling. Renin and ACTH have their own specific pre-analytical requirements, but the standard, widely emphasized practice to prevent rapid in vitro changes is chilling ammonia immediately after collection.