What effect does refrigeration have on PT samples longer than 24 hours?

Refrigeration of prothrombin time (PT) samples for longer than 24 hours can lead to alterations in the coagulation factors, specifically factor VII, which is sensitive to storage conditions. When PT samples are refrigerated for an extended period, these factors can become partially degraded or change in their activity, often resulting in a prolongation of the PT results.

This effect occurs because cold temperatures can affect the stability and activity of certain proteins involved in the coagulation cascade. Since factor VII has a relatively short half-life (approximately 6 hours), its degradation in refrigerated samples is particularly impactful. This leads to a less effective pathway for thrombin formation and, as a result, a prolonged PT.

In summary, refrigeration of PT samples for longer than 24 hours is known to prolong the PT results due to the impact on the stability and activity of key coagulation factors.