Comparison of the use of body mass index percentiles and percentage of ideal body weight to screen for malnutrition in children with cystic fibrosis

Background: The Cystic Fibrosis Foundation (CFF) recommends using the percentage of ideal body weight (%IBWCFF) and body mass index percentiles (BMIp) to assess weight-for-height status and to screen for malnutrition. Objective: The objective was to, examine the agreement and discrepancy between the use of %IBWCFF and BMIp for screening malnutrition. Design: Data from 13 021 children reported to the 2000 CFF Patient Registry were analyzed. Results: In children of average stature (ie, height-for-age between the 25th and 75th percentiles) and aged <10 y, %IBWCFF corresponded closely to BMIp, and the prevalence of underweight estimated by %IBWCFF < 90% was similar to that by BMIp < 15th percentile. However, in children with short stature (ie, height-forage < 25th percentile), %IBWCFF reflected significantly better weight-for-height status than did the BMIp, whereas the opposite trend was observed in children with tall stature (ie, height-for-age > 75th percentile). Such discrepancies averaged 8-12 percentage points when BMIp was reexpressed to the same unit and scale as %IBWCFF Consequently, the prevalence of underweight estimated by %IBWCFF < 90% was significantly lower (7.3%) than that estimated by BMIp < 15th percentile (25.7%) in children with short stature, whereas the opposite trend was found in children with tall stature (47.7% and 14.4%, respectively). Additional analyses showed that BMIp was more sensitive to, and had stronger associations with, the percentage of predicted forced expiratory volume in 1 s. Conclusion: Compared with BMIp, %IBWCFF underestimated the severity of malnutrition in children with short stature and overestimated the severity of malnutrition in children with tall stature.