Postoperative delirium (POD) remains one of most common complication in elderly patients receiving surgery, which is characterized by acute and fluctuating change in the level of cognition and consciousness. Previous studies have shown that surgery and anesthesia-induced acute peripheral inflammation and neuroinflammation may lead to delirium after surgery. Systemic Inflammation Response Index (SIRI) is a more easily accessible and comprehensive inflammation marker derived from monocyte, neutrophil, and lymphocyte count, which has been investigated to assess the prognosis of cancer and infectious diseases. However, the relationship between SIRI and POD has yet to be investigated.

Inclusion Criteria:

• Hospital stay of at least 3 days
• More than or equal to 65 years old
• Elementary education or above
• Able to communicate
• American Society of Anesthesiologists (ASA) physical status I-III
• Mini-mental State Examination (MMSE) scores ≥ 22 points

Exclusion Criteria:

• Preoperative delirium
• Severe mental diseases
• Autoimmune-illness, chronic infectious diseases, nervous system diseases and malignancies

Eligible patients were scheduled to undergo elective orthopedics surgery under spinal anesthesia with an expected hospital stay of at least 3 days and received postoperative patient- controlled analgesia. Delirium was evaluated at the preoperative visit, then twice daily for 3 days following surgery. Cognition assessment was conducted at preoperative visit, 24 h after surgery and 72 h after surgery. Delirium assessments were performed before 10 AM and after 5 PM in the ward. The confusion assessment method (CAM) questionnaire and cognition assessment method for MMSE were performed by the qualified doctor who trained with CAM and MMSE training manual.