Which of the following is NOT a specimen that requires chilling?

Prothrombin time is indeed the correct answer because it is a test that does not require chilling of the specimen prior to processing or analysis. Prothrombin time tests are typically used to evaluate the coagulation system and assess how well and how quickly blood clots. Specimens for this test can be transported at room temperature, as the coagulation factors being assessed are stable under these conditions for a limited time.

In contrast, tests such as ammonia, gastrin, and lactic acid require the specimen to be chilled to maintain their integrity and accuracy. Ammonia levels can rise rapidly at room temperature due to cellular metabolism, making it important to keep the sample cold. Gastrin, which is sensitive to temperature fluctuations, also needs to be chilled to prevent degradation of the hormone. Lactic acid levels can increase due to anaerobic metabolism if the sample is left at room temperature, so chilling is necessary to accurately assess lactic acid levels in the blood. Thus, prothrombin time stands out as the only test in this list that does not necessitate chilling, making it the correct choice.