Definition and criteria

For appropriately identifying metformin users in this study, we defined people who visited medical care and received a physician’s prescription for metformin under the coverage of Taiwan’s Health Insurance Program. In this study, we defined major inpatient surgery as surgical procedures requiring general, epidural or spinal anesthesia and index surgery with hospitalization for >1 day. Low-income status was defined as having a low income within 2 years before surgery. According to the regulations from the Ministry of Health and Welfare in Taiwan, people with low-income status were qualified to have the registration fee and medical copayment waived when visiting outpatient, emergency, and inpatient medical care. The criterion of low income was defined by local city or county governments. For example, a person living in Taipei (the capital of Taiwan) with a monthly income of less than US$500 (1 Taiwanese dollar is equal to US$30.324) and immovable possessions with a value of less than US$244 031 per household was considered to have a low income. In Taiwan, there were 144 863 low-income households and 304 470 low-income people in 2019. The definition of low income varies with urban and rural areas because of the local living conditions.

The International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) administration codes and physicians’ primary diagnoses were used to identify diabetes (ICD-9-CM 250), coexisting medical conditions (within the preoperative 24 months) and postoperative complications (that occurred during the index admission) for surgical patients.^{11 24} These medical conditions were determined from medical claims for the 24-month preoperative period and included hypertension (ICD-9-CM 401–405), mental disorders (ICD-9-CM 290–319), ischemic heart disease (ICD-9-CM 410–414), chronic obstructive pulmonary disease (ICD-9-CM 491, 492 and 496), hyperlipidemia (ICD-9-CM 272.0, 272.1 and 272.2), liver cirrhosis (ICD-9-CM 571.2, 571.5 and 571.6), heart failure (ICD-9-CM 428), alcohol-related illness, renal dialysis (administration codes D8 and D9), and Parkinson’s disease (ICD-9-CM 332). We defined alcohol-related illnesses, including alcoholic psychoses (ICD-9-CM 291), alcohol dependence syndrome (ICD-9-CM 303), alcohol abuse (ICD-9-CM 305), alcoholic fatty liver (ICD-9-CM 571.0), acute alcoholic hepatitis (ICD-9-CM 571.1), alcoholic cirrhosis of the liver (ICD-9-CM 571.2), and alcoholic liver damage (ICD-9-CM 571.3). Postoperative complications included postoperative bleeding (ICD-9-CM 998.0, 998.1 and 998.2), pneumonia (ICD-9-CM 480–486), septicemia (ICD-9-CM 038 and 998.5), urinary tract infection (ICD-9-CM 599.0), deep wound infection (ICD-9-CM 958.3), stroke (ICD-9-CM 430–437), acute myocardial infarction (ICD-9-CM 410), acute renal failure (ICD-9-CM 584) and pulmonary embolism (ICD-9-CM 415).

In this study, we examine the number of surgical procedures in every hospital in 2008–2013 and then categorized the surgical volume of hospital into three groups: low (the lowest tertile of surgical volume), moderate (the second tertile of surgical volume), and high (the highest tertile of surgical volume). In the National Health Insurance Program, the coverage of payment included all physician specialties of outpatient care, inpatient care, and emergency care. During the 24-month period before the index surgery, diabetes and coexisting medical conditions were defined as patients had at least two visits of medical care with physician’s primary diagnosis. The 30-day postoperative mortality was calculated as death occurred within 30 days after the time point of surgical procedure included the period of during and discharge of index surgical admission. The complications after surgery during the index surgical admission were considered as secondary outcomes. The length of hospital stay (more than 1 day), medical expenditures (US dollars), and intensive care during the index surgical admission were also compared between patients who did and did not use metformin preoperatively.