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Table 6 covariates used to adjust statistical models in at least one study

From: Assessing the hospital volume-outcome relationship in surgery: a scoping review

Patient characteristics Hospital characteristics Clinical conditions Severity Details of the disease Details of the surgery Details of the hospital stay Post-operative events
Age (patient, donor, recipient, gestational age) Hospital size (number of beds) Laboratory results (albumin, anaemia, hyperlipidaemia, natremia, and creatinine) The All Patient Refined Diagnosis Related Group score Time to treatment (door-to-balloon time, waiting time, etc.) Type of procedure and approach (laparoscopic, open, percutaneous, minimally invasive, etc.) Admission source (nursing facility, home, transfer within hospital, or ambulance) Adverse effects
Substance abuse/consumption (alcohol, illicit drugs, tobacco, etc.) Hospital ownership or type (private or public sector) Signs of shock on admission (heart rate, hypotension, Glasgow Coma Scale, Japan Coma Scale, hemodynamic instability, ECG) The Charlson-Comorbidity Index [32] Tumour characteristics (site, shape, size, stage, grade, differentiation status, histological subgroup, clinical-TNM, pre-therapeutic-TNM, extent, metastasis, distance from critical organs) Concomitant procedures (additional resections, lymphadenectomy, and valve surgery) Distance (from home, from previous facility, to the closest high-volume hospital, total distance travelled, etc.) Resection status (complete resection of positive margins)
Sex or gender Hospital teaching status or level (trauma centre, centre of excellence, reference centre, etc.) Heart disease (atrial fibrillation, LVEF trend, NYHA class [33]) The Elixhauser score [34] Injury severity and mechanism, trauma site Details of anaesthesia (substance used, duration, and local or general) Cost (total or out-of-pocket) Complications
Language Hospital location (urban, rural, city, state, country, region) Lung disease (chronic obstructive pulmonary disease, asthma, chronic lung conditions) The American Society of Anesthesiologists score (ASA) [35] Multiple and/or synchronous tumours Treatment modality (resection, laser, stenting, bariatric procedure, etc.) Admission status (scheduled or emergency) Post-operative radiotherapy or chemotherapy
Body mass index (or birthweight or obesity) or body surface area Number of licensed practical nurses per occupied bed Liver disease (cirrhosis or failure) Self-reported comorbidities Aneurysm characteristics (diameter and rupture) Risk Adjustment for Congenital Heart Surgery [36] Discharge type (transfer, recovery unit, nursing home, etc.)  
Geographic information (city, state, country, zip code, population, density) Number of registered nurses per occupied bed Renal disease (chronic or acute renal failure, creatinine, or dialysis) The modified Kellgren-Lawrence score [37] Specific scores:
International Federation of Gynecology & Obstetrics [38],
Furhman [39],
Injury Severity Score [40],
Kidney Donor Risk Index [41])
Surgical indication Length of uninterrupted institutional care (LUIC)  
Economic status or income (self-reported, zip-code-based, quartiles)   Chronic diseases (diabetes and depression) The Score for Neonatal Acute Physiology [42] Lymph node status and lymphadenectomy Surgeon’s specialty Period of admission (the year and/or month, on a weekday vs. the weekend)  
Educational level (self-reported, parents’ level, zip-code-based)   Pre-operative medical treatment (neo-adjuvant chemotherapy or radiotherapy, immunosuppressive therapy, aspirin, beta blocker, biopsies, statins) Pre-operative patient-reported outcome measures Benign or malignant tumours Time interval between diagnosis and surgery Total length of stay  
Social security status (or primary payer)   Previous surgery (coronary surgery, aortic surgery, total hip/knee replacement, or carotid revascularization)   Diagnosis and/or aetiology Intraoperative blood volume loss and/or transfusion requirement Specific length of stay (LUIC, ICU, pre-surgery unit)  
Race or ethnicity   Previous or active cancer   Date of diagnosis Oncological issues (concomitant examination and result, microscopic or macroscopic negative margins) Ambulatory status  
   Family history of disease   Number of diagnoses   ICU stay  
     Primary tumour site    
  1. ECG Electrocardiography, LVEF left ventricular ejection fraction, NYHA New York Heart Association, TNM tumour nodule metastasis classification, ICU intensive care unit, LUIC length of uninterrupted institutional care