Diagnosing obesity by body mass index in chronic kidney disease: an explanation for the "obesity paradox?".

Although obesity is associated with poor outcomes, among patients with chronic kidney disease (CKD), obesity is related to improved survival. These results may be related to poor diagnostic performance of body mass index (BMI) in assessing body fat content. Accordingly, among 77 patients with CKD and 20 controls, body fat percentage was estimated by air displacement plethysmography (ADP), skinfold thickness, and body impedance analysis. Defined by BMI ≥30 kg/m(2), the prevalence of obesity was 20% in controls and 65% in patients with CKD. Defined by ADP, the prevalence increased to 60% among controls and to 90% among patients with CKD. Although sensitivity and positive predictive value of BMI to diagnose obesity were 100%, specificity was 72%, but the negative predictive value was only 30%. BMI correctly classified adiposity in 75%. Regardless of the presence or absence of CKD, subclinical obesity (defined as BMI <30 kg/m(2) but excess body fat by ADP) was often missed in people with low lean body mass. The adjusted odds ratio for subclinical obesity per 1 kg of reduced lean body mass by ADP was 1.14 (95% CI: 1.06 to 1.23; P<0.001). Skinfold thickness measurements correctly classified 94% of CKD patients, but bioelectrical impedance analyzer-assessed body fat estimation did so in only 65%. Air displacement plethysmography-, skinfold thickness-, and bioelectrical impedance analyzer-assessed body fat all provided reproducible estimates of adiposity. Skinfold thickness measurements may be a better test to classify obesity among those with CKD. Given the low negative predictive value of BMI for obesity, our study may provide an explanation of the "obesity paradox."