Thirty-day outcomes after bariatric conversion operations: a national analysis of 94,828 cases.

BACKGROUND: Conversion bariatric operations are increasing and now comprise nearly 10% of procedures nationally, yet their 30-day risk compared with primary operations remains poorly characterized at the pathway level. OBJECTIVES: To estimate the adjusted excess 30-day serious adverse event (SAE) risk of conversion versus primary bariatric operations of the same final anatomy, and to identify which pathways carry the highest risk. SETTING: National Metabolic and Bariatric Surgery Accreditation and Quality Improvement Program (MBSAQIP) Participant Use File (PUF), 2020 to 2024. METHODS: We performed anatomy-stratified logistic regression with marginal standardization on 949,507 bariatric operations (94,828 conversions). The primary outcome was a 17-component composite SAE. Adjusted risk differences (RDs) per 1000 were computed for sleeve gastrectomy (SG), Roux-en-Y gastric bypass (RYGB), and biliopancreatic diversion with duodenal switch (DS)/single-anastomosis duodeno-ileal (SADI) strata, and a within-conversion model identified associated factors. RESULTS: Conversion was associated with higher adjusted 30-day SAE risk across all anatomies: SG (RD 18.2 per 1000 [95% confidence interval, CI: 15.0-21.3], number needed to harm [NNH] 55), RYGB (22.1 [19.5-24.7], NNH 45), and DS/SADI (17.5 [9.4-25.7], NNH 57). The excess was concentrated in utilization events (reoperation, reintervention, and readmission) and selected technical complications, such as leak, organ-space infection, and bowel obstruction. Among conversions, a prior RYGB carried the highest risk (adjusted odds ratio [aOR]: 2.54 versus a prior sleeve). The excess showed no detectable change across study years. CONCLUSIONS: Conversion operations carry a clinically meaningful excess 30-day SAE risk that is greatest after a prior gastric bypass. Conversion status should be treated as a distinct risk category rather than benchmarked against primary procedures alone.