Final answer:
A royal blue-topped tube is used for medical laboratory tests, specifically for a gastric function assessment that determines the presence of free hydrochloric acid in the stomach using Diagnex blue.
Step-by-step explanation:
The purpose of a royal blue-topped tube is typically associated with medical laboratory tests that require specific conditions for the specimen. In the context of the information provided, the royal blue top indicates that it is used for tests such as a gastric function assessment with Diagnex blue. The process involves the administration of Diagnex blue to assess the presence of free hydrochloric acid (HCl) in the stomach. If the intensity of the blue color in the test urine matches or exceeds the 0.6 mg standard of Azure A, it indicates the presence of free HCl. Lower intensities suggest conditions such as hypochlorhydria or achlorhydria.