The Predictive Value of Simple Inflammatory and Nutritional Indices for Postoperative Complications in Bariatric Surgery Patients. A Study Protocol for a Single-Centre Retrospective Study from a Referral Hospital

Background/Objectives: Nutritional depletion and systemic inflammation are common in patients living with obesity undergoing metabolic bariatric surgery (MBS) and may influence the risk of early postoperative complications. Composite indices derived from routine blood tests—combining markers of protein/immune reserve and inflammatory burden—offer an inexpensive means of preoperative risk stratification, but their comparative performance in MBS has not been well defined. The primary objective is to compare the prognostic performance of seven preoperative indices for early postoperative complications after MBS; the secondary objective is to estimate and rank the incremental prognostic value of each index over baseline clinical risk factors. Methods: This protocol describes a single-centre retrospective cohort study of approximately 700 consecutive adults who underwent sleeve gastrectomy, one-anastomosis gastric bypass, Roux-en-Y gastric bypass, single-anastomosis duodeno-ileal bypass with sleeve gastrectomy or single-anastomosis sleeve-ileal bypass, including revisional procedures, between 1 January 2023 and 28 June 2026; data extraction will begin only after submission of this protocol. The Prognostic Nutritional Index (PNI), Neutrophil-to-Lymphocyte Ratio (NLR), Platelet-to-Lymphocyte Ratio (PLR), Systemic Immune-Inflammation Index (SII), Systemic Inflammation Response Index (SIRI), C-reactive protein-to-Albumin Ratio (CAR) and Glucose-to-Lymphocyte Ratio (GLR) will be calculated from complete blood count, albumin, C-reactive protein and fasting glucose values. The last preoperative sample is the exposure; early postoperative values (postoperative days 1–3) will be examined in a separate exploratory landmark analysis. The primary outcome is any complication of Clavien–Dindo grade II or higher within 30 days of surgery; readmission within 30 days of surgery and prolonged hospital stay will be analysed as secondary healthcare-utilisation outcomes. The indices will be compared by their optimism-corrected incremental discrimination over a baseline clinical model, using penalised logistic regression with bootstrap internal validation and calibration assessment. Conclusions: This protocol outlines a pragmatic approach to comparing seven low-cost, routinely available nutritional–inflammatory and inflammatory indices for risk stratification before MBS. Indices showing promising performance will require external, ideally multicentre, validation before they can be considered for clinical decision making.