Higher Body Mass Index Is Associated with a Lower Incidence of Postoperative Urgent-Start Hemodialysis in Hospitalized Patients with CKD: A Nationwide Retrospective Cohort Study in Japan

Background: A rapid decline in renal function following surgery can lead to postoperative acute kidney injury (PO-AKI) and the need for urgent dialysis initiation, which is related to shorter time to early mortality compared to chronic dialysis initiation. Body mass index (BMI) has been suggested to be a risk factor for PO-AKI. However, there are no studies assessing the impact of BMI on the incidence of urgent start dialysis. Methods: We utilized data from inpatient databases in Japan (2020–2023), covering nearly 50% of all hospital admissions nationwide. The study included 2,814 non-dialysis-dependent CKD (NDD-CKD) patients and 154,037 non-CKD patients aged 18–89 years who underwent lower gastrointestinal surgery. Patients were categorized into quartiles based on BMI: Q1 (low BMI), Q2 (normal BMI), Q3 (high BMI), and Q4 (obese BMI). The primary outcome was the incidence of urgent hemodialysis following surgery. Multivariate logistic regression analysis was performed to adjust for baseline characteristics. Results: Among NDD-CKD patients, postoperative urgent dialysis was most common in the low BMI group (Q1: 28.1%) compared to normal (Q2: 26.4%), high (Q3: 17.5%), and obese BMI groups (Q4: 18.1%). Adjusted logistic regression showed a significantly lower incidence in the obese group (p < 0.001), with adjusted odds ratios (aORs) of 0.93 (95% CI: 0.883–0.973) for Q4 and 0.95 (95% CI: 0.903–0.994) for Q3, relative to Q2. In contrast, among non-CKD patients, the highest incidence occurred in the obese group (Q4: 0.51%) versus the normal BMI group (Q2: 0.20%), with a significantly higher risk in Q4 (p < 0.001) after adjustment. Conclusion: Higher BMI is associated with a significantly lower incidence of postoperative urgent dialysis in non-dialysis-dependent CKD patients undergoing lower gastrointestinal surgery. In contrast, higher BMI correlates with worse renal outcomes in non-CKD patients. These findings suggest that elevated BMI may contribute to improved renal outcomes following surgery, depending on the presence or absence of CKD.