What is a common indicator that a blood sample has clotted?

When a blood sample clots, one of the most distinguishing features observed is the presence of a gel-like substance. This gel forms as a result of the coagulation process, where fibrinogen is converted into fibrin strands that weave together to form a solid mass, trapping blood cells and other components within it. This gel-like structure is particularly noticeable when the sample is allowed to sit undisturbed after collection.

In contrast, while other indicators may affect the appearance or temperature of a sample, they are not reliable signs of clotting. For example, feeling a sample that is cold could result from external factors like refrigeration, and a color change or opacity might arise from other causes, such as the type of plasma or serum present or the cellular content of the sample. Thus, the presence of that distinct gel-like substance provides a definitive indicator of clotting in the sample.