Sepsis is a life-threatening condition caused by the body's overwhelming response to infection, and it remains a leading cause of death in intensive care units. Even when standard treatment targets such as blood pressure are met, blood flow to the small vessels of the skin and other tissues may remain impaired. This "hidden" poor perfusion may not be detected by routine monitoring, yet it may be linked to worse outcomes.

The perfusion index is a simple number obtained from the standard pulse oximeter already placed on a patient's finger. It reflects blood flow in the peripheral tissues, requires no additional procedure, and causes no discomfort.

This prospective observational cohort study will examine whether the perfusion index, and especially how it changes over the first 24 hours in the intensive care unit, can help predict death within 28 days in adults with sepsis or septic shock. Capillary refill time and skin mottling score will be recorded as additional bedside measures of peripheral perfusion.

No treatment will be changed for the purpose of this study, and no additional blood samples or invasive procedures will be performed. All care decisions remain with the treating team. Measurements will be taken at intensive care admission and at 6 and 24 hours. Participants will be followed for 28 days, with survival status also recorded at 90 days.

The study will be conducted at two centers. A prediction model will be developed in the first center and then tested, without modification, in patients enrolled at the second center.