POASAC Study (Preoperative Anemia Severity and Classification Study)

The goal of this observational study focuses on the effect of different preoperative anemia status on perioperative complications in patients under moderate- to high-risk surgery. Different anemia types conclude iron-deficiency anemia(IDA), anemia of chronic disease/anemia of inflammation(ACD/AI), and combined anemia.The main question it aims to answer is whether there is a difference in the incidence of complication related to oxygen supply and demand imbalance in patients with IDA, ACD/AI or combined anemia. Patients without preoperative anemia will be recruited as the control group for secondary outcome analysis, which is whether there is a difference in the incidence of complication related to oxygen supply and demand imbalance in patients with or without anemia.

Inclusion Criteria: * age 18 years or older * scheduled for elective moderate- to high-risk surgery other than intracranial and cardiac surgery * predicted surgery time longer than 1 hour and hospital stay longer than 24 hours * understand and agree to participate Exclusion Criteria: * pregnant * live at areas higher than 2,500 meters above sea level * with hematological diseases other than anemia * with severe comorbidities that might interfere outcome assessment, like uremia, dementia, and so on * can't cooperate or communicate, or reject to sign informed consent form