Non-linear relationship between BMI and eGFR at admission in elderly patients with hip fractures: a retrospective cross-sectional study.

Objective: Previous studies have found that the relationship between changes in BMI and renal function outcomes remains unclear, with both increases and decreases in BMI being associated with a decline in eGFR, especially in the elderly hip fracture population. Therefore, this study aimed to investigate the association between BMI and eGFR upon hospital admission in elderly patients with hip fractures, to better understand the underlying pathophysiological connections and provide a basis for clinical patient management.

Methods: We conducted a retrospective cross-sectional study involving 1,417 elderly patients with hip fractures, collecting clinical data such as BMI and eGFR at admission. Generalized additive models, smooth curve fitting, and two-piecewise linear regression models were used to explore the linear and non-linear relationships between BMI and eGFR.

Results: A total of 1,417 patients with hip fractures were included, with females accounting for 73.04%. The mean age of the patients was 77.57 ± 8.73 years. The mean eGFR was 80.23 ± 21.76 mL/min/1.73 m², and the mean BMI was 22.04 ± 3.25 kg/m². After adjusting for age, sex, fracture classification, number of comorbidities, smoking, hypertension, coronary heart disease, diabetes mellitus, white blood cell, lymphocyte, neutrophil, hemoglobin, red cell distribution width, albumin and globulin, a significant correlation between BMI and eGFR was still observed (β: -0.72, 95% CI: -1.04 to -0.40). Trend analysis after categorizing BMI into four groups showed that the decrease in eGFR increased progressively with increasing BMI. Non-linear analysis revealed an inflection point at a BMI of 27.5 kg/m². To the left of the inflection point, for every 1 kg/m² increase in BMI, eGFR decreased by 0.91 mL/min/1.73 m², while no correlation was found to the right of the inflection point.

Conclusion: There is a non-linear relationship between BMI and eGFR at admission in elderly patients with hip fractures, with a clear inflection point. This suggests that BMI should be considered an indicator for renal function risk assessment when managing these patients, and that targeted renal protective measures should be considered for patients in different BMI ranges.