Percent body fat and prediction of surgical site infection

Obesity is a risk factor for surgical site infection (SSI) after elective surgery. Body mass index (BMI) is commonly used to define obesity (BMI >or=30 kg/m(2)), but percent body fat (%BF) (obesity is >25%BF [men]; >31%BF [women]) might better predict SSI risk because BMI might not reflect body composition.

This prospective study included 591 elective surgical patients 18 to 64 years of age from September 2008 through February 2009. Height and weight were measured for BMI. %BF was calculated by bioelectrical impedance analysis. Preoperative, operative, and 30-day postoperative data were captured through interviews and chart review. Our primary, predetermined outcomes measurement was SSI as defined by the Center for Disease Control and Prevention.

Mean %BF and BMI were 34+/-10 and 29+/-8, respectively. Four-hundred and nine (69%) patients were obese by %BF; 225 (38%) were obese by BMI. SSI developed in 71 (12%) patients. With BMI defining obesity, SSI incidence was 12.3% in nonobese and 11.6% in obese patients (p = 0.8); Using %BF, SSI occurred in 5.0% of nonobese and 15.2% of obese patients (p < 0.001). In univariate analyses, significant predictors of SSI were %BF (p = 0.005), obesity by %BF (p < 0.001), smoking (p = 0.002), National Nosocomial Infections Surveillance score (p < 0.001), postoperative hyperglycemia (p = 0.03), and anemia (p = 0.02). In multivariable analysis, obese patients by %BF had a 5-fold higher risk for SSI than nonobese patients (odds ratio = 5.3; 95% CI, 1.2-23.1; p = 0.03). Linear regression was used to show that there is a positive, nonlinear relationship between %BF and BMI.

Obesity, defined by %BF, is associated with a 5-fold increased SSI risk. This risk increases as %BF increases. %BF is a more sensitive and precise measurement of SSI risk than BMI. Additional studies are required to better understand this relationship.